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- 1 Structure
- 2 Anatomy
- 3 Morphology
- 3.1 Base
- 3.2 Ribs
- 3.3 Margin
- 3.4 Foil
- 3.5 Petiole
- 3.6 Color
- 3.7 Hairs
- 3.8 Surface
- 4 Evolution of the leaf
- 5 Distribution on the stem (phyllotaxis)
- 6 Related items
- 7 Other projects
- 8 External links
The structure of a leaf, at the macroscopic level, consists of four parts:
- the sheath (enveloping structure at the level of the insertion on the stem)
- the stipules (appendages present at the base of the leaf, recognized by the leaves due to the lack of axillary buds)
- the petiole (the stem of the leaf)
- the lamina, also called flap (the flat part of the leaf, has a very variable size and shape in relation not only to the different species but also in the same individual, in relation to environmental conditions).
It is rare for the leaf to have all four of its parts.
The ribs are the conductive system of the leaf.
The point where the petiole is attached to the stem is called the leaf axilla. Not all plant species produce leaves whose structure includes all the parts mentioned above in some species the petiole is absent (leaves sessile, for example in maize), in other plants the lamina can be very small. In the pea plant, the lamina is transformed into a cirrus, while the photosynthetic function is performed by the stipules. In celery, the so-called stalk is actually a large, fleshy sheath.
In current plants the leaves are formed in an acropetal series from the leaf drafts present in the caulin apex and deriving from the tunic. In the early stages of development all the cells of the draft divide intensively. Subsequently we distinguish a basal portion that slows down the series of divisions and an apical portion that is still active.
- The leaf sheath derives from the basal area (not always present)
- The stipules (not always present) originate from the contact area between the basal and apical part of the draft.
- The petiole and the lamina differ from the apical area (the growth of the lamina usually precedes that of the petiole).
Based on the anatomy of the leaf blade we can distinguish:
- double-sided leaves (dorso-ventral): plagiotropic posture eg. in most Dicotyledons. In this case we distinguish an upper (adaxial) and a lower (abaxial) page that are different from each other
- equifacial leaves (isolateral): with orthotropic posture eg. in most Monocotyledons. In this case the two pages are not distinguished, which have the same structure, eg. Gramineae, narcissus.
- centric leaf (needle-like leaf of conifers) thus defined for the position of the cribrovascular bundles is a case of the previous type
- unifacial leaves, which have a single visible face, this is because the leaf folding on itself along its major axis allows the union of the two ends of the abaxial face and in doing so shows only the upper part of the lamina. They have a tubular or axial appearance, eg. some monocots, such as onion, iris, aloe.
A leaf is considered an organ of the plant, typically it consists of the following tissues:
- an epidermis that covers the upper and lower surface. The top is often covered with the cuticle, a waxy substance (cutin) that makes the leaf impermeable.
- a mesophyll, consisting of 2 parenchyma. A parenchyma a palisade above and one incomplete below. The palisade one is full of chloroplasts while the incomplete one, in addition to containing chloroplasts, is characterized by wide intercellular spaces.
- a characteristic arrangement of the ribs (the cribro-vascular bundles). The xylem it is located above and includes the vessels for the supply of water and salts coming from the roots. The phloem. is located below and includes elongated cells modified to form tubes (called cribrosi) that allow the transport of products of photosynthesis to the sites of use or accumulation.
- the stomata which are microscopic openings arranged on all the herbaceous parts of the plants, in particular on the leaves, their function is to maintain the gaseous exchange with the outside, in particular the escape of water vapor and the entry of oxygen and anhydride carbonic.
The leaves that present this anatomy are called bifacial or dorsoventral (common among dicotyledons). The face (or page) facing up is called superior, or adaxial, or ventral, the face facing down is called inferior, or abaxial, or dorsal. If the two faces of the lamina are equivalent, we speak of equifacial leaves (eg grasses). In the onion (liliaceae) the leaf takes on a tubular appearance, therefore it exposes only one face to the outside, while the other remains internal and not exposed: unifacial leaf. The leaf of the conifers has a very thin, albeit consistent, lamina and is called needle-like.
Based on the disposition of the stomata, there will be: 1. hypostomatic leaves (if the stomata are in the lower face, in the bifacial leaves) 2. epistomatic leaves (if the stomata are in the upper face, in the bifacial floating leaves of aquatic plants, such as the water lily ) 3. amphistomatic leaves (if stomata are distributed on each face, in the equifacial leaves) 4. astomatic leaves (if they are devoid of stomata, for example the leaves of submerged plants).
The leaves can be distinguished and classified according to the conformation and structure of their constituent parts.
Chlorophyll disappears in autumn, before the leaves fall: thus the other pigments become visible.
- roped: falls within the 90th
- dull: base extended for more than 90 °
- acute: base extended for less than 90 °
- cuneata: base extended for less than 15 °
- kidney shape: the petiole attaches to the center of the leaf
- truncates: flat base at about 180 °
- asymmetrical: the point of attachment on the petiole is out of phase
- irregular: asymmetrical base that attaches to the same point as the petiole
Pinnate: They are very scattered around the leaf
Parallel: They are parallel to each other
Curves: They tend to curve towards the end
Branched: They are heavily branched and spread equally across the leaves
The ribs are conductive vessels present in the foliar epidermal and allow the passage of all substances.
The leaves can be classified by the type of margin:
- whole, the margin has no incisions
- wavy or sinuous, the margin has pronounced undulations
- toothed, the margin has acute protrusions or serrations directed towards the outside of the leaf
- doubly toothed, the margin has a main dentition on which a smaller dentition appears (e.g. cannabis)
- toothed-spiny, the teeth are prolonged with long points
- serrated, the margin has acute protrusions facing the apex of the leaf
- crenate, the protrusions are similar to teeth but with a rounded outline
The leaves are also distinguished by the depth of incision of the margin:
- lobed, the incisions are few and deep, but do not reach the middle of the lamina
- fool, the incisions are deeper than in the lobed one and reach midway between the lamina and the median rib
- set, the incisions reach the median rib
- roncinato, the lobes are curved backwards towards the base of the leaf
- elliptical: apex and round base with maximum width in the center.
- ovata: maximum width in the lower third part
- obovata: maximum width in the third upper part
- lanceolate: very long and narrow lamina
- rhomboid: rhombus-shaped
- heart-shaped: round foil in the shape of a heart
- composed pinnate: compound leaf
- lobed pinnate: leaf with penninervia lobes
- lobed webbed: palmate leaf with lobes
- pedunculated: with petiole
- sessile: there is no stalk
- crushed: flattened
- with stipules: with small leaves at the base
- with sheath: the sheath wraps around the branch
- amplessicaule: the sheath completely covers the branch
- color: same color on both pages
- color: different color on the two pages
- hairless: withour hair
- pubescent: small hairs
- tomentosis: abundant hair
- leathery: hard and thick
- tomentose: velvety
- rough: rough surface
From an evolutionary point of view, the leaves differentiated later than the stem. The oldest corm plants that lived about 380 million years ago (Psilopsida), of which only fossil remains remain, were rootless and had a dichotomously branched stem without leaves.
According to Zimmermann (1956) the first terrestrial plants would have been formed only by a little branched stem and by a part immersed in the soil attributable to a root. This theory starts from the observation of a Devonian macrofossil (400 million years ago), belonging to the genus Rhynia. The stem initially would have given rise to all identical branches, of a dichotomous type. The portion of the stem above the last branch is defined teloma. Even the leaves would have derived from the modification of the dichotomous ramifications. The formation of the leaves would have occurred in stages according to the following scheme:
1) one of the telomas develops more than the others, assuming the appearance of a branch while the second remains reduced, 2) the small telomas, arranged on several planes, flatten themselves, arranging themselves on a single plane, 3) each teloma flattens and becomes it blends with the others arranged on the same plane thanks to the new formation of laminar portions of fabric.
If a teloma remains isolated it has a microfilm (eg gymnosperm needles) If more telomas merge, we have a macrophilus (e.g. leaves of a few gymnosperms and all angiosperms). A further evolution of this structure involves the anastomosis between the cribro-vascular bundles belonging to the different telomas.
Leaves can be classified by how they position themselves along the stem of a plant. The distribution is usually characteristic of a species. The leaves can be:
- opposite, 2 leaves brought in a single knot in opposite position.
- opposite decussate, leaves opposite where each pair is rotated 90 degrees from the previous one.
- alternating or alternating, leaves arranged one per node and oriented alternately on one side and the other.
- verticillate, three or more leaves that are inserted on the stem at a single node.
- rosette or radicals, leaves that grow at the base of the stem to form the so-called rosette.
The stamens constitute the androceum (male fertile part) of the Angiosperm flower and can be free or gathered in groups.
pea. '' The stipule is an appendix that differs at the base of the petiole in the leaves.
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From A to Z
TAXIA. - In ethology and physiology of behavior we designate with this term (from the Greek ἡ ταξις = disposition) the mechanisms by which an animal assumes and maintains a determined position of an axis of the whole body (or of an appendix) with respect to a stimulating field . In the t. we distinguish an afferent component that records the body position in the field, the process of central evaluation of the afferent, and, finally, an efferent component that leads to final motor coordination.
Depending on its spatial structure, a natural stimulating field can be given: a) by agents whose lines of force are practically parallel (e.g., sunlight or moon, gravitational field, Earth's magnetic field, atmospheric, river, sea currents ) b) from gradient structures (e.g. diffusion gradients of a chemical substance in air or water) c) from sources of stimuli more or less strictly localized and, typically, of configurational structure, such as salient objects in the environment, fixed (elements of the landscape near or far) or mobile (silhouette of the prey, the predator, the species mate, etc.). The same stimulating field can be characterized by more than one of the listed properties.
The concept of t. it is often applied to the mechanism underlying the oriented actions of the whole animal, but it is also valid for the oriented movements of parts of the body (partial t.). Here we refer, however, above all to the former. While in the cinèsi (see the entry, in this App.) The orientation is a secondary, statistical effect, due to a succession of actions or reactions not oriented with respect to the field, in the t. orientation is directly achieved by their own mechanism. The common use of the term often implies locomotion in the field (it is said, for example, that an animal is phototactic positive if it proceeds towards a light source) but, strictly speaking, the t. it includes only the processes related to the torsional movement with which the animal reaches the oriented state (O. Koehler) and maintains this state (H. Schöne). When an animal turns towards the prey and stares at it, for example, the subsequent locomotion may also be lacking, but the control of the body position with respect to the stimulus, that is, the t., Is already complete.
Subsequent locomotion is part of a different position control mechanism in space which has been called "elasia" (from ἠ ἔλαςις = driving) (J. A. Chmurzinsky, R. Jander). The t. controls the directional (rotational) orientation, elasia the translational orientation, ie the distance to be covered.
T. and elasia are independent, but they can be closely related, when the animal moves in a stimulating field, either by controlling only the course (see, for example, the case of many littoral animals that know how to return to the shore by the shorter), both by controlling course and distance (for example in migratory birds): in the latter case the orientation is called "vectorial".
The t. they can be classified: a) according to the sensory modalities operating b) according to the geometric arrangement of the animal in the field c) according to the mechanisms.
Following the first criterion, photo - thermo - chemogeum - hygro - galvano - magnetotaxia, etc., can be distinguished. the most common forms. Today it is customary to designate with particular terms also the use of special references within a given sensory modality: we thus speak of astrophylaxia for the use of celestial references (sun, moon, stars), of polarotaxia for the use of polarized light, of scototaxy for orientation towards or away from dark shapes, of tigmotaxia for the research and use of mechanical contact stimuli, of rheotaxy for orientation with respect to liquid currents, of anemotaxia for the wind, of osmotaxia or phonotaxia for the use of olfactory or acoustic references, and so on. These are - obviously - purely descriptive terms, sometimes convenient: it should be noted that, in some cases, t. it is used as a pure synonym of orientation, in which the truly tactical component is not always distinguished from the kinetic ones (see also cinèsi).
Considering the geometry of the orientation for t. (rotatory) it should be remembered that while a terrestrial animal orientates itself mainly by rotation around the dorsal-ventral axis (antero-posterior if the station is erect), an animal in motion or in flight normally has two other rotational possibilities: around the dorsal-ventral axis, the rotation around the antero-posterior axis and the transverse axis is added, which - in marine terms - would be said for roll and pitch. Having said this, a "basic" or "direct" orientation can be distinguished - from a geometric point of view - in which, whatever the rotation axis of the t., The longitudinal axis is turned and maintained towards or away from the source of stimuli (and / or the most intense area of the gradient) from a "transverse" orientation, in which the longitudinal axis forms a fixed or variable angle different from o ° and 180 ° with the direction of incidence of the stimulus. Examples of direct orientation provide the numerous cases of animals turning towards or away from a light source (positive or negative phototaxy), upwards or downwards (geotaxy), etc. The transverse orientation is given, for example, by the so-called "photodorsal and photoventral reactions" of numerous aquatic forms (the animal swims by turning its belly or back to light), by the lateral phototaxia of grasshoppers and other insects that rotate up to to expose one side to sunlight, from menotaxia (see below), in which the animal can assume and maintain any angle of orientation with respect to the direction of the stimulus.
The problem of the taxie mechanism (and of orientation) arises between the past century and the present one with J. Loeb's theory of tropisms (1888-1918). For Loeb "the movements caused by light or other factors (there is still no explicit distinction between locomotion and torsion) seem to the layman the expression of a will and a purpose present in the animal, while, in reality, the animal is forced to go where his legs lead him: since the conduct of animals consists of forced movements "(1918).
The Loebian explanation of the so-called positive "phototropism" of an animal is schematically the following: equal stimulation on the symmetrical photoreceptors (of a bilateral symmetry animal) produces identical excitations in the central nervous system and therefore equal "tension" in the symmetrical muscles, unlike unequal stimulation. In the first case the animal orients itself and moves straight towards the light, in the second it is first forced to rotate until the equality of excitation is restored.
Although opposed by many (G.D. Mast, 1911, for example), these concepts dominated for a long time in the first half of the century.
A. Kühn (1919) bases his system on the Loebian conception, however distinguishing between tropisms and taxies. He reserves the term "tropism" for the oriented dispositions that certain sessile animals assume, as a result of differential accretions in front of light or in the gravitational field. I am instead t. the elementary orientation reactions of free or sessile animals, through movements, also for Kühn similar to reflex movements of the whole individual. However, compared to Loeb, he has a much broader and more complex conception of such reactions, which takes greater account of a phenomenology that is already quite varied for his time. Among the t., Kühn in fact distinguishes the phobotaxis, today considered between the cinèsi (see this entry), and the topotaxias, reactions oriented in a stimulating field, corresponding to the definition of t. introduced here at the beginning. They are topotaxias according to Kühn: tropotaxia, telotaxia, menotaxia and mnemotaxia. The first three are categories defined (see below) on the modalities of reception of the stimulus at the level of the sense organs, the last - completely changing register - as an orientation dependent on information learned and memorized.
The system subsequently proposed by G.S. Fraenkel and D.G. Gunn (1940-1961) represents a progress and a regression with respect to the Kühnian one. Mnemotaxia is correctly eliminated, a category based on a heterogeneous criterion compared to the others and on a characteristic that can, on the other hand, be present in all forms of taxia. Unfortunately, Fraenkel and Gunn also base their system on heterogeneous criteria, partly causal, partly purely descriptive.
The classification adopted here includes among the t: clinotaxia, tropotaxia, telotaxia and menotaxia. The so-called "transverse reactions" of Fraenkel and Gunn can be traced back to tropo- or telotactic mechanisms.
Clinotaxia presupposes pendular or rotational movements with which the stimulating field is systematically explored and during which successive excitations are compared with each other. For clinotaxia, an asymmetric or unequal receptor, capable of registering variations in intensity, is also sufficient.
Classic is the example of the photoclinotaxy of apod larvae of certain Brachiceri Diptera (Musca, Calliphora, Lucilia) which possess a group of photoreceptor cells in a pocket of the cephalopharyngeal skeleton. In the photonegativity phase they orient themselves and proceed away from the light, with regular pendulations of the head of symmetrical extension until the subsequent stimulations are identical: a more intense excitation while the animal is turned to the right (from whatever direction it comes) causes a more extended torsion to the left and vice versa, so that the uniformity of subsequent stimulations is automatically re-established. The positive phototactic behavior of the Flagellates of the genus Euglena is described as photoclinotactic, in which the exploration of the light field is carried out with the continuous rotation of the body around the longitudinal axis. Since the photoreceptor (paraflagellar body) is shielded on one side by the so-called "stigma" (a pigmented organelle), with a lateral incidence light, it is subsequently found in light and in shadow: the animal, through the flagellum, turns as long as it can move towards the light without the periodic change of excitation of the receptor, which in this case has directional sensitivity.
Osmoclinotaxy has been described in bees experimentally deprived of an antenna: they know how to find a source of fragrant food to which they have been trained, exploring the olfactory gradient both with oscillations of the antenna and of the whole body (if the only antenna is immobilized): while in the case of fly larvae the torsion tends to minimize, here - on the contrary - it tends to maximize the signal and the route taken therefore becomes a collision with the stimulus source.
Kühnian tropotaxia takes up and broadens Loeb's concept of tropism. The tropotactic mechanism presupposes a pair of antagonistic sensory units (receptors or groups of receptors), among which the motor action of the animal tends to restore and maintain a balance of excitation. In latero-lateral tropotaxia, the mechanism is based on at least two symmetrical sensory units, either separated into equal sense organs, or confined to two antagonistic sections of the same organ: they can be either only sensitive to variations in the intensity of the stimulus or also possess a directional sensitivity.
In both cases the excitations coming from them are simultaneously compared in the centers and, in the case of a difference, these provoke an asymmetrical motor action by means of the musculature of the two sides and therefore a torsion. The "positive" animal rotates, for example, on the dorso-ventral axis (color change) towards the side of the receptor more stimulated, the negative one in the opposite direction (avoids the stimulus): it finds and maintains the direction of a gradient anyway and / or the direction of a stimulus source.
Characteristic operational criteria of tropotaxia are: 1) the forced circular movements of the animal when one of the two antagonistic units is excluded (towards the non-excluded side if the animal is positive and vice versa) 2) the trajectory according to the so-called "law of the resultant ": between two sources of stimulus (eg between two lights) the animal arranges itself according to a resulting direction, evaluating its incidence and intensity (fig. 1).
There is no doubt that many cases of natural orientation can be formally described and interpreted on this basis: animals that behave phototropotactically are, for example, common in many groups (Insects, Crustaceans, Annelids). A clear example of chemotropotaxia is found in Planarias (Platelminti, Turbellari): if a capillary tube is presented to a planaria, filled with a nutritional liquid and folded into a fork so that from both terminal openings it diffuses an identical chemical stimulation that goes to excite the chemoreceptors of the auricles (the two cephalic expansions) and move it slowly and symmetrically, the animal moves according to the trajectory traveled by the midpoint of the fork, without deciding on one or the other side, like the donkey of Buridano (fig. 2).
A chemo- (osmo-) tropotaxia is also demonstrated in Bees: a worker with her antennae immobilized in a crossed position, in front of a Y-tube, in which - let's say - an attractive odor comes from the right branch, systematically rotates towards the left branch: in fact it is the left antenna that is most stimulated and the resulting nervous message will therefore be of "twisting to the left" (fig. 3). In this way it is possible to determine exactly what minimal difference in concentration between right and left can still be perceived and, by gluing the antennas with the tips at different distances, it can be shown that osmotropotactic orientation is possible as long as the distance between the two antennal points is greater than 2 mm: below this distance osmodynotaxia takes over. Thermotropotactic can probably be considered the discovery of warm-blooded prey by the rattlesnakes, which have two thermosensitive dimples in front of their eyes. Hygrotropotaxia is described for some Insects (eg in Tribolium).
In all these cases the tropotactic equilibrium conditions a longitudinal direct orientation towards or away from the source, but if the physiologically antagonistic receptors are rostrum-caudal, the animal can assume a transversal orientation: certain aquatic beetle larvae, for example. (Acilius, Dytiscus provided with stemmata or larval ocelli) assume a transverse position with different angles with respect to the light coming from above and maintain it, counteracting the rolling by means of tropotactic interaction between right and left stemmata and pitching by interaction of the rostral stemmata and caudals.
Telotaxia is always a direct orientation towards a source of stimuli that is achieved without the condition of the excitation balance between antagonistic receptor areas: there exists in the receptor or receptors of the telotactic animal a particular area (usually frontal) called "area of fixation ". Each stimulation outside this area causes a torsion that leads the stimulus to hit the same area. Once this condition is reached, any stimulation of other areas is inhibited and the animal fixes and, possibly, moves with the body or with one of its appendages towards the stimulus source, even with only one of two equal receptors. Telotactic orientation usually presupposes reticular or mosaic construction receptors, consisting of more or less numerous juxtaposed sensory units, each of which transmits its own local signal (compound eyes of Arthropods, retina of vertebrates, for example). The fixation area can be preformed and fixed or, in certain cases, have a different and centrally determined position from time to time.
Orienting stimuli in telotaxia are often objects of a configurational nature (Gestalten) such as the prey, the species companion, the source of food.
Operational criteria of telotaxia are, in contrast to tropotaxia, the absence of circular movements when one of the even receptors is excluded and, when the animal is exposed to two sources of stimulation, the orientation towards one or the other.
Example of phototelotaxia in laboratory conditions in front of localized light sources are known for several Crustaceans. As an example of a phototelotactically directed mechanism, the capture of prey with the tongue of a frog or a chameleon can be indicated, or the blow of the raptorial forelimbs with which a praying mantis also grabs the prey.
While tropo- and telotaxia are characterized by seeking and maintaining one or two basic fixed positions (towards and / or away from the stimulus source) without terms of passage, menotaxia is extraordinarily versatile: the animal can maintain for a time more or less along any angle between a body axis (typically the anteroposterior axis) and the direction of the stimulus source. In natura compare in campi di stimolazione a linee di forza parallele (luce solare, lunare, campo gravitazionale, correnti aeree). Come la telotassia, richiede recettori di tipo reticolare e può essere ricondotta a un meccanismo simile a quello telotattico, quando si ammetta che il punto di fissazione venga spostato per intervento centrale in un'area del recettore di volta in volta diversa.
Sono noti esempi di foto- geo- e anemomenotassia. In natura la fotomenotassia solare è molto diffusa. Molti Insetti (Formiche, Coleotteri, bruchi di Lepidotteri), vari Molluschi e Crostacei sono capaci di mantenere una rotta rettilinea per un certo tempo, facendo un angolo costante con il sole. Tale comportamento può avere un significato biologico, allorché aumenta per l'animale la probabilità di uscire rapidamente da un'area sfavorevole che non offre altri punti di riferimento vicini (per es. una distesa sabbiosa uniforme). L'animale, che può variare a caso il proprio angolo di orientamento con il sole, non tiene conto della variazione di azimut dell'astro ( fotomenotassia non cronometrica ).
La fotomenotassia diventa cronometrica quando l'animale fa uso di un meccanismo di misurazione del tempo ( orologio interno ) e può mantenere una direzione di rotta costante anche basandosi su un riferimento apparentemente mobile come il sole, variando regolarmente e continuamente l'angolo menotattico. L'orientamento solare cronometrico è una delle forme più frequenti di orientamento degli animali ripari (Crostacei, Aracnidi, Insetti, taluni Pesci e Anfibi), che - mediante tale meccanismo - riescono a mantenersi entro la loro zona di elezione, o a ritrovarla con un percorso di minimo dispendio, perpendicolare alla direzione della riva (L. Pardi e F. Papi). Taluni animali ripari (Crostacei, Aracnidi) sono pure capaci di orientamento lunare cronometrico . La menotassia cronometrica è alla base dei meccanismi con cui le Api ritrovano e segnalano alle compagne una sorgente di cibo redditizia (K. v. Frisch). L'operaia segnalatrice, infatti, nell'oscurità dell'alveare, assume (durante la danza-linguaggio) un angolo con la direzione di gravità che corrisponde - secondo un codice semplice - a quello utile in quel momento per raggiungere la meta dall'arnia, valendosi del sole. L'operaia ricevente ritraduce l'angolo con il riferimento gravitazionale in un angolo con il sole. Ambedue gl'individui operano una trasposizione fotogeomenotattica .
L'orientamento menotattico cronometrico con il sole è infine uno dei meccanismi più importanti per il mantenimento della rotta di migrazione degli Uccelli e dei Pesci ed è uno dei componenti del comportamento di homing (per es. nell'homing dei Colombi viaggiatori). Forme speciali di orientamento menotattico sono l'orientamento con la luce polarizzata celeste che - in ombra - può sostituire quello solare. Poiché il pattern di luce polarizzata del cielo si sposta solidalmente con il sole, anche questo meccanismo comporta una variazione cronometrica dell'angolo. Come altra forma speciale di orientamento menotattico può essere considerato anche l' orientamento stellare , dimostrato ormai per varie specie di Uccelli migratori: in questo caso il sistema di riferimento è rappresentato da una configurazione complessa (costellazione o insieme di costellazioni). Il meccanismo può essere anche non cronometrico, se gli animali (come sembra dimostrato per alcuni Uccelli) si basano sulle costellazioni circumpolari, che permangono costantemente nella medesima area del cielo.
La classificazione delle t. secondo i meccanismi operanti qui adottata sulla base del sistema kühniano modificato da G.S. Fraenkel e D.G. Gunn e da altri, ha avuto e ha sicuramente ancor oggi un valore in quanto permette un inquadramento utile di molti fenomeni dell'orientamento animale: riviste sintetiche recenti (F.C. Creutzberg, 1975) ne fanno ancor oggi uso corrente e non vi è dubbio che molti casi possano essere formalmente descritti e interpretati nei termini di clino- tropo- telo- e menotassia.
Vi è tuttavia oggi la tendenza a una critica approfondita di queste categorie da vari punti di vista ed è presumibile che, con il progresso della conoscenza sui meccanismi neurologici di base, tuttora assai incompleta, profondi mutamenti concettuali si renderanno opportuni. Alcune critiche recenti non riguardano propriamente i concetti relativi al meccanismo interno delle t., ma rilevano unicamente come essi interessino solo aspetti parziali del processo di orientamento. L'orientamento traslatorio, per es., è ignorato e trascurata è del pari la fase, iniziale ed essenziale, dell'identificazione dei riferimenti adeguati per l'orientamento, diversi nei differenti cicli funzionali (trofico, riproduttivo, associativo e competitivo, ecc.). Il concetto di t., d'altronde, semplifica oltre misura in quanto - di solito - considera l'orientamento secondo una sola modalità sensoriale (fotochemo- geotassia, ecc.), mentre in molti casi almeno, il processo si basa su una simultanea valutazione e interpretazione di informazioni provenienti da canali sensoriali diversi: eccezioni a parte, questo campo è ancora largamente inesplorato. Altre obiezioni investono però direttamente i concetti dei vari meccanismi tattici. Viene sottolineata in molti casi la difficoltà di certe distinzioni apparentemente rigorose: per es. la separazione fra tropotassia e telotassia è talvolta impossibile persino da un punto di vista operativo, quando un animale privato di uno dei recettori antagonisti, dopo una prima fase di ruotazione coatta, compensa perfettamente la perdita e si orienta verso la sorgente "telotatticamente " con un solo recettore. Soprattutto poi - ed è questa l'obiezione fondamentale - si rimprovera al sistema tradizionale delle t. di concepire ancora l'orientamento come una pura reazione (un riflesso dell'animale intero) dovuta essenzialmente a una certa struttura e/o disposizione dei recettori, in cui l'intervento dei centri è ridotto al rango di semplice trasformatore dell'eccitazione sensoriale nell'efferenza motoria. Ora tale concezione può corrispondere effettivamente ad alcuni casi limite, mentre di solito la situazione è ben altrimenti complessa. Frattanto, ha conseguito un notevole sviluppo l'interpretazione cibernetica delle t. che, pur senza considerare i dettagli della loro base morfologica e funzionale, fornisce una rappresentazione chiarificatrice dei rapporti causali fra gli elementi del sistema operante nell'orientamento e precisa i concetti di t. come reazione e azione . Si distinguono, da questo punto di vista, t. senza regolazione , puramente reattive, in cui, data una certa disposizione centrale, l'orientamento decorre "cieco", una volta localizzato lo stimolo, senza ulteriore controllo (per es. la cattura della preda in una rana), da t. a regolazione , confrontabili a un dispositivo di controllo con retroazione negativa. Qui il "valore di riferimento" ( index value , Sollwert) dato dai centri, viene continuamente confrontato, in un ipotetico elemento comparatore, con il "valore attuale" ( actual value , Istwert) registrato come afferenza nell'elemento recettore. La differenza fra i due valori (errore) trasmessa all'elemento effettore viene trasformata nella torsione che tende a ridurre l'errore. Ora, lo scostamento fra valore attuale e di riferimento può avere due origini diverse: o è imposto da un disturbo esterno sulla posizione dell'animale, o si origina in un mutamento autonomo del valore di riferimento, per intervento centrale. Nel primo caso siamo ancora in presenza di una t. reattiva nel senso tradizionale, nel secondo caso di una t. che dev'essere propriamente definita come "azione di orientamento". In ambedue i casi, la t. non è "una funzione dello stimolo, cioè dell'afferenza come tale, ma dello scostamento dell'afferenza [dal valore di riferimento dettato] da una specifica disposizione centrale" (H. Schöne).
Così da un'interpretazione puramente reattiva della t., per cui l'animale è forzato ad andare dove le gambe o le ali lo portano, si è giunti a un'interpretazione attiva di molti fenomeni di orientamento, secondo cui l'animale valuta, momento per momento, gli scostamenti fra la propria disposizione interna e le informazioni sensoriali e trasforma questa valutazione nel comportamento adeguato.