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KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1904. 



As we cannot deny the extreme probability, perhaps 

 we may say the certainty, that all plants now living have 

 been descended from some primitive form, we can find in 

 the history of different races the enormous effects which 

 long-continued struggle for successful adaptation to a 

 changing environment can achieve. The effect of change 

 upon a single individual may be, indeed, must be, slight ; 

 but long-continued influence upon long series of descen- 

 dants brings about a marked ctunulative effect, and 

 though we see little change in a generation, we are obliged 

 to admit relatively enormous modification in the course 

 of time. But though we can see little alteration in the 

 individual, we may argue backwards and realise that no 

 great change could occur in a race except by modifications 

 of successive indi\'iduals of it. We must, therefore, look 

 minutely to the individual to see what the properties are 

 which in long years can effect such modifications of both 

 form and structure as we find. 



We have then to study what we may call the adapta- 

 tion of the organism to its environment. At the outset, 

 we must admit that such adaptation can take place only in 

 two ways. Possibly, all plants whose constitutions are 

 not in harmony with the changed conditions will perish, 

 leaving more fortunate ones to carry on the race. 

 This postulates that the plants of any particular genera- 

 tion are themselves varying slightly in their physiological 

 properties. Possibly, on the other hand, the individual 

 organism is possessed of a power of appreciating changes 

 in its surroundings and of modifying its own behaviour 

 accordingly. It may well be that both these hypotheses 

 are to a certain extent true, and that they are co-operating 

 to bring about the results we see. 



There are strong grounds for accepting the latter of the 

 two views as playing a very prominent part in the 

 changes of the past. We can see certain phenomena 

 occurring under our own eyes which are capable of in- 

 terpretation in the way suggested, which, indeed, are 

 inconsistent with any other hypothesis. A plant acted 

 upon by a certain definite external influence modifies its 

 way of behaviour in an equally definite manner. It is 

 difficult to deny to the plant the power of perceiving the 

 influence brought to bear upon it. The effect of the in- 

 fluence is technically called a stimulus, and the percep- 

 tion of a stimulus by the plant is known as a soisatioii. 

 We have two factors then to consider, one external, the 

 other internal, to the plant. 



A more complicated question arises here. Is the percep- 

 tion of a stimulus, is a sensation, to be interpreted as 

 implying any kind o{ consciousness ? We have a stimulus, 

 we have a response. What can we say of the interpreta- 

 tion of the one by the plant which makes it bring about 

 the other ? The problem is very difficult to speak with con- 

 fidence upon in the present state of knowledge. The 

 human mind shrinks at once from taking the affirmative 

 view. No doubt, in the higher sense in which we interpret 

 the word, no consciousness can have part in a vegetable 

 organism, for this sense implies ihuus^ht. It is difficult 

 to suggest that a purposeful response implies any kind of 

 volition. These operations are the immediate functions 

 of the well-organised and most highly-developed nervous 

 centres of the highest animals. But certain facts can be 

 adduced which, at any rate, hint at the existence of such 

 a limited consciousness as implies an appreciation of the 

 nature of the surroundings. 



To discuss this question at any lenj^th would, however, 

 take us beyond the purpose of this article. We must 

 confine ourselves to the question of stimulus and sensation 

 as far as we can see them both at work in the course of 

 ordinary vegetable life, leaving the full interpreta- 



tion of the relation between them to be set aside for the 

 present. 



The nature of a stimulus first concerns us. We may 

 take it for granted that there may exist for every plant, 

 at any rate theoretically, a condition of adjustment when 

 it is in absolute harmony with its environment — when 

 temperature, illumination, moisture, rest, and whatever 

 else affects it, are perfectly as the organism wants them, 

 and when consequently its life is lieing regulated to the 

 utmost advantage. Such a condition can be only 

 momentary in any case, for the surroundings are in a 

 constant state of change in many of these particulars, and 

 the living substance of the plant is also exhibiting con- 

 tinual motility. For the maintenance of health, or even 

 of life, it is essential that variations in the one shall be 

 adequately responded to by variations in the other. The 

 impossibility of securing indefinitely such a continual 

 adjustment of relations is the cause of the cessation of 

 life. 



Such an alteration of the environment constitutes a 

 stimulus. It may affect the plant in a hundred ways, 

 causing various methods of response, and various degrees 

 of intensity of response. 



There are, however, other factors influencing its 

 life which are not so easily realised by observation. 

 Changes may arise in the condition of the living sub- 

 stance of the plant, set up perhaps by disturbances in its 

 interior. The normal cause of chemical change associated 

 with the nutritive processes may undergo a marked change 

 in consequence of an alteration of the distribution or the 

 direction of the stream of food in the plant's interior. 

 Injury to the body of the plant may involve a re-distribu- 

 tion of energy or of material within it, which may have 

 far-reaching effects upon the course of the vital processes. 

 Variations in the supply of food, which may range be- 

 tween absolute starvation and over-engorgement, may 

 produce very great changes not only in the outer life of 

 the plant, but in the substances it produces in the course 

 of its nutritive processes, and in the energy which it 

 liberates. An insufficient supply of oxygen may provoke 

 an almost entirely new series of chemical changes in 

 connection with the production of such energy. These 

 various factors and many others which might be quoted 

 are to. be regarded as stimuli, some of them internal no 

 doubt, but all equally real and equally well appreciated 

 by the plant as the more obvious external ones just de- 

 scribed. Even more obscure stimulations may arise 

 from chemical changes in the living substance itself, 

 leading to a series of responses which, as they do not 

 appear immediately related to \isible stimuli, are often 

 called automatic. 



To appreciate more fully the part played by stimulation 

 in the life of a plant, we may briefly consider a few of its 

 more obvious forms. Consider the lateral incidence of 

 light upon a growing seedling or young plant. If the 

 latter is placed so that one side of its stem is more bril- 

 liantly illuminated than the opposite, a curvature soon 

 appears in the part that is actively growing. This is of 

 such a nature and takes place to such an extent as to 

 cause the axis of the plant to take up a position in which 

 it is parallel to the direction of the incident rays. It 

 manifests itself in some cases very slowly, in others com- 

 paratively rapidly. This response to the stimulus of un- 

 equal illumination on its two sides is not confined to the 

 stems of seedlings, but may be seen to a greater or less 

 degree in parts of many adult plants. It is a matter of 

 common observation that geraniums grown in a windoAv 

 all bend their steins and petioles towards the illuminated 

 side. 



