November 14, 1901] 



NATURE 



which I have spoken, or with Sorghum, as in Fig. 5. If one 

 of these is supported by its seed with its stem projecting 

 freely in the horizontal plane, the gravitation stimulus makes 

 it bend upwards until the tip is vertical, when the stimulus 

 ceases to act and the curvature comes to an end. If the con- 

 ditions are reversed, if the seedling is supported in a horizontal 

 position by its tip, while the seed projects freely, the result 

 is at first the same, though finally it comes to he strikingly 



Fig. 5. — Seedling -S"(i^;^/i«;«v supported by their tips in horizontal g!ass 

 tubes. (From the Aunah of Botany.) 



different. The basal end of the seedling is carried upwards by 

 the curvature of the stem ; but according to the theory we are 

 testing, the tip of the seedling is the only part of the plant which 

 feels the gravitational stimulus, and the tip of the seedling 

 remains horizontal in spite of the curvature of the stem. There- 

 fore the tip of the seedling is not freed from stimulation as it 

 was in the first case, where the curvature brought the tip into 



Fig. 6.-:-.\ bean-rt 



vhich had been supported by the tip ; the 

 us corresponds to that in Fig. 5. 



the vertical position. The horizontal tip therefore continues 

 to send commands to the stem to go on curving, in a way I can 

 best explain if I am allowed to make the plant express its sen- 

 sations in words. The tip says to the stem, " I am horizontal, 

 therefore you must bend upwards"; and when this order has 

 been obeyed the tip says, " It is of no use, I am still horizontal — go 

 on bending." The result is that the stem curls up into a spiral 

 like a cork.screw or a French horn, as shown in Fig. 5. I have 



NO. 1672, VOL. 65] 



also been able to get the same result with the roots of beans 

 and peas, as shown in Fig 6.' 



These unfortunate plants are in the position of a convict on 

 the treadmill ; their movements are, from their own point 

 of view, absolutely ineffectual and meaningless. The results 

 are, however, of some importance from our point of view, 

 since they give clear support to the theory which I have 

 now attempted to place before you, namely, that the perci- 

 pient region is at the tip of the Setaria seedling or of the bean 

 root (as the case may be), and that by what corresponds to a 

 reflex action the stimulus perceived by the tip is transmitted to 

 the motor region. I think I may quote my father's words and 

 say that it is hardly an exaggeration to say that the tip acts like 

 the brain of one of the lower animals. 



I should like to add a few words on the question how far 

 the movement of plants can be placed under the general 

 laws deducible for the movements of animals. Unfor- 

 tunately, as soon as we attack this question we are liable to 

 enter regions where for the ignorant there are many pitfalls. 

 We are, in fact, face to face with the question whether in 

 plants there is anything in which we may recognise the faint 

 beginnings of consciousness, whether plants have the rudiments 

 of desire or of memory, or other qualities generally described 

 as mental. 



If we take the wide view of memory which has been set forth 

 by Mr. S. Butler - and by Prof Hering, we shall be forced to 

 believe that plants, like all other living things, have a kind of 

 memory. For these writers make memory cover the whole 

 phenomena of life. Inheritance with thein is a form of memory, 

 or memory a kind of inheritance. A plant or an animal grows 

 into the form inherited from its ancestors by passing through 

 a series of changes, each change being linked to the preceding 

 stage as the notes of a tune are linked together in the nervous 

 system of one who plays the piano. Or we may compare the 

 development of an animal or plant to the firing of a train of 

 gunpowder, which completes itself by a series of explosions each 

 leading to a new one. To use the language I have been em- 

 ploying, each stage in development acts as a signal to the next. 



In the same way the characteristic element in what is done 

 by memory or by that " unconscious memory " ' known as habit 

 is the association of a chain of thoughts or actions each calling 

 forth the next. 



What I wish to insist on is that the process I have called 

 action by signal is of the same type as action by association, and 

 therefore allied to habit and memory. The plants alive to-day 

 are the successlul ones who have inherited from successful 

 ancestors the power of curving in certain ways when, by acci- 

 dental deviations from their normal attitude, some change of 

 pressure is produced in their protoplasm. With the pianist the 

 playing of .^ has become tied to, entangled or associated with 

 the playing of B, so that the playing of A has grown to be a 

 signal to the muscles to play B : similarly in the plant the act of 

 bending has become tied to, entangled or associated with, that 

 change in the protoplasm due to the altered position. There is 

 no mechanical necessity that B should follow A in the tune ; the 

 sequence is owing to the path built by habit in the man's brain. 

 And this is equally true of the plant, in which a hereditary habit 

 has been built up in a brain-like root-tip. 



The capacities of plants of which I have spoken have been 

 compared to instincts, and if I prefer to call them reflexes it is 

 because instinct is generally applied to actions with, something 

 of an undoubted mental basis. I do not necessarily wish it to be 

 inferred that there can be nothing in plants which may possibly 

 be construed as the germ of consciousness — nothing psychic, to 

 use a convenient term ; but it is clearly our duty to explain the 

 facts, if possible, without assuming a psychological resemblance 

 between plants and human beings, lest we go astray into 

 anthropomorphism or sentimentality, and sin against the law 

 of parsimony, which forbids us to assume the action of higher 

 causes when lower will suffice. 



The problem is clearly one for treatment by evolutionary 

 method — for instance, by applying the principle of continuity.'' 

 Man is developed from an ovum, and since man has conscious- 

 ness it is allowable to suppose that the speck of protoplasm from 

 which he develops has a quality which can grow into conscious- 

 ness, and by analogy that other protoplasmic bodies, for 

 instance those found in plants, have at least the ghosts of similar 



1 F. D.-irwin, in Pri 

 ■i "Life and Habit,' 

 3 Mr. S. Butler's te 

 ■I See James Ward, 



. Cambridge Phil. So. 



Naturalisin and Ag: 



