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Trewavas décrivait les plantes comme capables d’intention.
Mais j’avais en tête la formulation de Jacques Monod : le fait
d’attribuer un but ou un projet à la nature est contraire aux fon-
dements de la méthode scientifique. Pour Monod, étudier la
nature scientifiquement veut dire ignorer la possibilité d’inten-
tion. J’ai rappelé ce postulat à Trewavas, en ajoutant qu’il sem-
blait l’avoir outrepassé.
Il a répondu d’un ton railleur:
– Eh bien, je ne sais pas combien de gens croient vraiment
Jacques Monod sur ce point. D’abord, son idée ne s’appli-
quait pas vraiment aux humains. Pour moi, elle semblait dévi-
taliser la vie. Elle semblait indiquer que la vie est uniquement
gouvernée par le hasard. Or les animaux peuvent prévoir.
Tout comme nous d’ailleurs. Et pour moi, la plasticité ne peut
être que prévoyance, car elle représente l’aptitude à s’ajuster
aux conditions environnementales particulières que l’on ren-
contre. Sans cette capacité, l’accommodation aux circons-
tances ne pourrait être optimale. Pour la plante, la plasticité
revient à prévoir les conditions possibles dans lesquelles elle
va se trouver.
– Alors, comment la plante fait-elle pour décider ? lui ai-je
demandé.
Trewavas a répondu qu’il avait réfléchi de nombreuses années
à la question. En 1990, ses collègues et lui avaient fait une per-
cée. Ils étaient en train d’étudier comment les plantes perçoivent
les signaux et transmettent 1′ information de manière interne.
À l’aide de manipulations génétiques, les chercheurs ont intro-
duit dans des plants de tabac une protéine qui les faisait luire
quand le niveau de calcium augmentait à l’intérieur des cellules.
lls avaient émis l’hypothèse que l’altération de concentration
cellulaire en calcium était l’un des moyens principaux par lequel
les plantes percevaient les événements extérieurs. À leur grande
surprise, ils avaient découvert que les plants de tabac réagis-
saient immédiatement au toucher. Bien que le tabac ne soit pas
connu pour être sensible au toucher, une petite caresse suffisait
pour provoquer chez les plantes modifiées une émission de
lumière induite par l’augmentation de calcium dans leurs cel-
lules. Trewavas était ébloui par la rapidité de la réaction : « Sa
vitesse était telle qu’elle était à la limite de ce que nous pou-
vions mesurer. Alors que je vous ai dit que les plantes ne réagis-
sent qu’en termes de mois et d’années, dans ce cas-ci, elles
répondaient en quelques millièmes de secondes à un signal dont
nous savions qu’il aurait plus tard un effet morphologique.
Quand on touche une plante régulièrement, sa croissance ralentit
et la plante devient plus épaisse. »
Trewavas savait que les neurones humains eux aussi produi-
sent une augmentation interne de calcium lorsqu’ils transmettent
de l’information. Après avoir vu la vitesse à laquelle les plantes
réagissaient au toucher, il avait commencé à penser à leur intelli-
gence. Bien que les plantes n’aient pas de neurones, s’était-il dit,
leurs cellules utilisent un système de signalisation de même
type, de sorte qu’elles ont peut-être la capacité de calculer et de
prendre des décisions.
En l’écoutant, je réalisais qu’il avait vécu en direct les
changements qui avaient bouleversé la biologie contempo-
raine au cours des dernières décennies. Il s’était ouvert à
l’idée de l’intelligence dans la nature. Pour un scientifique
occidental, cela représentait un pas audacieux. Je connaissais
des indigènes d’Amazonie pour qui l’intelligence des plantes
allait de soi. Mais dans la culture occidentale, les gens qui
attribuent de l’intelligence aux plantes ont longtemps été ridi-
culisés. Jusqu’à maintenant, les scientifiques, et en particulier
les botanistes, ont évité les mots « intelligence des plantes ».
Je voulais en savoir plus. Comment sa manière de penser
avait-elle changé ? J’ai insisté pour qu’il me donne plus de
détails.
En désignant d’un geste les piles de documents qui jalon-
naient son bureau, il me répondit que pendant des dizaines
d’années, il avait énormément lu sur des quantités de sujets. Il
me révéla certains aspects de sa méthode de travail. « La
famille se plaignait parce que je restais assis sur une chaise à
réfléchir dans le vide. Je trouvais que c’était nécessaire. Les
idées ne viennent pas seulement en lisant. Il faut s’éloigner des
livres, s’étendre, s’asseoir, marcher, laisser les choses tourner
dans sa tête. Et s’il y a une situation que j’apprécie entre toutes,
c’est bien d’essayer de résoudre un problème dans ma tête.
Y a-t-il des connexions nouvelles à établir ? Et je trouve que
c’est seulement grâce aux longues périodes où l’on ne fait rien
d’autre que penser, que tout à coup les faits commencent à se
mettre en place. Et ils viennent regroupés dans une combinai-
son intéressante qui vous permet d’entrevoir des possibilités
quant à ce que les plantes peuvent faire. » Il dit que la notion
d’intelligence chez les plantes lui était venue de cette manière.
L’intelligence en général était un sujet qui le captivait depuis de
longues années. Ainsi, lorsqu’il avait vu la connexion entre les
plantes et le calcium, cela l’avait inévitablement conduit à réflé-
chir à propos d’intelligence.

Plants have at least three kinds of propagating electrical signals. In addition to a sustained wound potential (WP) that stops a few millimeters from dying cells, these sig- nals are action potentials (Aps) and slow wave potentials (SWPs). All three signals consist of a transient change in the membrane potential of plant cells (depolarization and subse- quent repolarization), but only SWPs and Aps make use of the vascular bundles to achieve a potentially systemic spread through the entire plant. The principal difference used to differentiate SWPs from Aps is that SWPs show longer, delayed repolarizations. Unfortunately, SWP repolarizations also show a large range of variation that makes a distinction difficult. SWPs and Aps do differ more clearly, however, in the causal factors stimulating their appearance, the ionic mechanisms of their depolarization and repolarization phases as well as the mechanisms and pathways of propagation. The depolarizations of a SWP arise with an increase in turgor pressure cells experience in the wake of a hydraulic pressure wave that spreads through the xylem conduits after rain, embolism, bending, local wounds, organ excision and local burning. The generation of Aps occurs under different environmental and internal influences (e.g. touch, light changes, cold treatment, cell expan- sion) that – mediated through varying generator potentials  trigger a voltage-dependent depolarization spike in an all-or-nothing manner. While Aps and Wps can be triggered in excised organs, SWPs depend on the pressure difference between the atmosphere and an intact plant interior. High humidity and prolonged darkness will also suppress SWP signaling. The ionic mechanism of the SWP is thought to involve a transient shutdown of a P-type H + -ATPase in the plasma membrane and differs from the mechanism underlying Aps. Another defining characteristic of SWPs is the hydraulic mode of propagation that enables them  but not Aps – to pass through killed or poisoned areas. Unlike Aps they can easily communicate between leaf and stem. SWPs can move in both directions of the plant axis, while their amplitudes show a decrement of about 2.5% cm −1 and move with speeds that can be slower than Aps in darkness and faster in bright light. The SWPs move with a rapid pressure increase that establishes an axial pressure gradient in the xylem. This gradient translates distance (perhaps via changing kinetics in the rise of turgor pressure) into increasing lag phases for the pressure-induced depolarizations in the epidermis cells. Haberlandt (1890), after studying propagating responses in Mimosa pudica, suggested the existence of hydraulically propagated electric potentials at a time when only Aps were conceivable. It took a century to realize that such signals do exist and that they coincide with the characteristics of SWPs rather than those of Aps. Moreover, we begin to understand that SWPs are not only ubiquitous among higher plants but represent a unique, defining characteristic without parallels in lower plants or animals

Per trasportare informazioni da una parte all’altra del proprio corpo, un vegetale si avvale di segnali non solo elettrici ma anche idraulici e chimici. Dispone quindi di tre sistemi indipendenti, e a volte complementari, che funzionano sia a breve che a lungo raggio e sono in grado di raggiungere regioni della pianta distanti tra loro da pochi millimetri a decine di metri. Vediamo rapidamente come funzionano.
Il primo sistema, basato su segnali elettrici, è uno dei più usati ed è, in pratica, lo stesso impiegato dagli animali e dall’uomo, anche se con qualche «personalizzazione vegetale». Per esempio abbiamo già detto che le piante non hanno nervi, cioè non possiedono quei tessuti dedicati alla trasmissione dei segnali elettrici che gli animali utilizzano per la conduzione degli impulsi nervosi. A prima vista si tratta di un bel problema: come fare a spedire in giro segnali elettrici senza avere tessuti dedicati? Le piante hanno trovato una soluzione molto funzionale: per i brevi percorsi questi segnali passano da una cellula all’altra tramite semplici aperture presenti sulla parete cellulare, chiamate «plasmodesmi» (dal greco plásma, struttura, e désma, legame); per i percorsi più lunghi (ad esempio quello dalle radici alle foglie) usano invece il «sistema vascolare» principale.
Come? Le piante non hanno un cuore, ma possiedono un sistema vascolare? Proprio così: al pari degli animali, anche i vegetali sono dotati di un apparato idraulico che serve principalmente per trasportare materiale da un punto all’altro dell’organismo e che funziona come un vero e proprio sistema vascolare, del tutto simile al nostro tranne per il fatto che è sprovvisto di una pompa centrale (cioè non ha un cuore, in accordo con la necessità di non possedere organi unici di cui abbiamo già ampiamente parlato). Le piante, dunque, hanno un apparato circolatorio che consente il trasporto dei liquidi dal basso verso l’alto e viceversa: una specie di sistema arterioso e venoso, che è detto «xilematico» quando funziona dal basso in alto e «floematico» quando i liquidi scorrono invece dall’alto in basso.
Lo xilema (dal greco xúlon, legno) è il tessuto di conduzione principalmente adibito al trasporto di acqua e sali minerali (ma vi transitano anche altre sostanze) dalle radici verso la chioma, mentre il floema (dal greco phloiós, corteccia) è il tessuto di conduzione che segue il percorso inverso, trasportando gli zuccheri prodotti dalla fotosintesi dalle foglie ai frutti e alle radici.
Lo scopo di questa circolazione interna è subito evidente se si pensa che l’acqua assorbita dalle radici è perduta per traspirazione dalle foglie in grande quantità, e deve quindi essere continuamente reintegrata, mentre gli zuccheri prodotti dalla fotosintesi – che rappresentano la principale fonte di energia della pianta – devono essere traslocati di continuo dal sito di produzione (le foglie) alle altre parti dell’organismo. Attraverso questo complesso sistema vascolare, i messaggi elettrici circolano in modo agevole e abbastanza rapido. Così, segnali che se fossero trasportati per via chimica impiegherebbero moltissimo tempo per arrivare a destinazione, possono viaggiare in poco tempo fra le radici e le foglie, trasportando messaggi urgenti quali quelli che riguardano lo stato idrico del terreno.
C’è poca acqua? Ce n’è molta? Le foglie, avvisate tempestivamente, si regoleranno di conseguenza.

Intelligence is a term fraught with dificulties in definition.
In part, the problems arise because of the human slant
placed on the use and meaning of the word. However,
although as a species we are clearly more intelligent than
other animals, it is unlikely that intelligence as a biological
property originated only with Homo sapiens. There should
therefore be aspects of intelligent behaviour in lower
organisms from which our superlative capabilities are but
the latest evolutionary expression.

Stenhouse (1974) examined the evolution of intelligence
in animals and described intelligence as `Adaptively
variable behaviour within the lifetime of the individual’.
The more intelligent the organism, the greater the degree of
individual adaptively variable behaviour. Because this
de®nition was used to describe intelligence in organisms
other than humans, it is a de®nition useful for investigating
the question in plants. Do plants exhibit intelligent
behaviour? The use of the term `vegetable’ to describe
unthinking or brain-dead human beings perhaps indicates
the general attitude.

However, in animal terms, behaviour is equated with
movement, and since plants exhibit little if any form of
movement, plant intelligence on that basis does not exist.
Although some higher plants exhibit rapid movements (e.g.
Mimosa pudica), these are exceptions rather than common-
place. Mimosa captures our attention because it operates on
a time scale similar to our own, and it is the difference in
time scales that frequently makes plants seem unmoving.
The use of time-lapse facilities has indeed indicated that
plants operate on very much slower time scales than our
own, but once observed in this way, movement is quite
clear.

In addition, the majority of multicellular plants, including
macroalgae, are sessile, the result of a decision several
billion years ago to gather energy and reducing potential via
photosynthesis. Since light is freely available, movement
has never been particularly critical to plant survival. Such
movement as has been observed is usually limited to less
complex plants such as blue-green algae. Rejection of that
(photosynthetic) decision by the primordial animal eukar-
yotic cell ensured that movement became critical to ®nd
food and mates. Once animals started to prey upon each
other, the development of highly differentiated sensory
systems and specialized nerve cells to convey information
rapidly between sensory tissues and organs of movement
was an inevitable consequence. The predatoryprey relation-
ship has acted as a positive feedback loop to accelerate
complex development and equally complex organ differen-
tiation in animal evolution (Trewavas, 1986b). Movement
is, however, the expression of intelligence; it is not
intelligence itself. Stenhouse (1974) regarded the early
expressions of intelligence in animals as resulting from
delays in the transfer of information between the sensory
system and the motor tissues acting upon the signals. The
delay enabled assessment of the information and modifica-
tion of information in the light of prior experience, and it
was that assessment that formed the basis of intelligence.
The key difference between plants and animals in the
Stenhouse (1974) definition is in the word `behaviour’.
Silvertown and Gordon (1989) have defined plant behaviour
as the response to internal and external signals. In plant
terms these are familiar growth and development phenom-
ena, such as de-etiolation,  flower induction, wind sway
response, regeneration, induced bud break/germination,
tropic bending, etc. Thus, a simple definition of plant
intelligence can be coined as adaptively variable growth
and development during the lifetime of the individual. To
add significance to this definition, time lapse shows that
virtually all plant movements are indeed the result of growth
and development.