INTRODUCTORY 17 



light reaches it, it bends slowly away from the illu- 

 minating ray; if anything comes into contact with its 

 tip, growth causes it to curve so as to leave the obstacle 

 on one side. The young root shows in these ways 

 certain sensitivities, reacting to the incidents of its 

 environment, and behaving as if it were possessed of 

 rudimentary perceptions of direction, illumination, and 

 contact. Other features of the environment also affect 

 it, particularly moisture. The shoot in its behaviour 

 shows similar phenomena, but its conduct when influ- 

 enced, or, as it is generally called, stimulated, by gravity, 

 light, or other disturbing causes, is as a rule the opposite 

 of that of the root. It grows upwards against gravity; 

 it curves towards and not away from light ; its behaviour 

 with regard to contact is not always uniform. Both 

 parts, however, show what we claimed at the outset as 

 one of its primal necessities, the power of adjusting 

 itself to changes in its surroundings. 



Consideration of the fourth requirement, the power to 

 reproduce itself, must be deferred for the present. 



We may now with advantage turn to the composition 

 of the plant and ask what is the distribution in it of the 

 living matter to which this behaviour is to be attributed. 



It is best to begin the study of this point by examin- 

 ing quite a young plant, or preferably the seed of a 

 plant, as the structure is then simple, 

 while it becomes very complex as the plant 

 grows. If we take a seed (Fig. 3) we find 

 it contains a young plant or embryo, in 

 which bv careful dissection we can make J^****^ ^* 



J , ,, FIG. 3. Section 



out a young root and a young shoot. The O f a seed, 

 shoot consists of a short axis, to which > embryo, 

 are attached either one or two leaves 

 known as cotyledons, with perhaps traces of more 

 leaves above them (Fig. 4). When we cut such a 

 young root or young shoot, we find that it is made up of 



B 



