588 LECTURE XXI. 



can only do so briefly and in the most general way, into the 

 significance of movement in the economy of the plant. It is, 

 in fact, only when we clearly apprehend that the various 

 movements of plants have some bearing upon the well-being 

 of the organism that we can approach the difficult subject of 

 the mechanism of those movements with any hope of under- 

 standing it. It is only from this point of view that we can 

 perceive the principle which alone can afford a clue to the 

 mechanism of movement, the principle, namely, that all 

 phenomena of movement induced by a stimulus are not the 

 expression of the direct action of the stimulus on the cells by 

 means of which the movement is performed, but are the 

 response of the stimulated organ. 



We will begin by discussing the most general of the 

 phenomena of movement, the fact that growth is more rapid 

 in darkness than in light. The question at once arises why 

 this should be so; for it is possible to conceive that just the 

 opposite should be the case, there being no a priori reason for 

 the existing relation between light and the rate of growth. 

 The answer is given by the following instance. A green 

 plant requires to be exposed to light of a certain intensity in 

 order that its metabolic processes may be carried on. If a 

 seed of such a plant happens to germinate in a shady spot 

 among bushes, the seedling will not receive light of sufficient 

 intensity to enable it to carry on its processes of constructive 

 metabolism, and the only means by which this can be attained 

 is that its stem should grow rapidly in length so as to raise its 

 leaves above the level of the surrounding bushes and expose 

 them to the sunlight. Clearly, then, it is of great importance 

 to the plant that, as we have seen, the growth of its stem 

 should be rapid in feeble light. When once the apex of the 

 stem has reached the light, there is no further need for rapid 

 elongation ; all that is then required is that its tissues should 

 be well differentiated, so as to give rigidity to the stem and to 

 ensure an adequate supply of water to the upper parts of the 

 plant, and that its leaves should be fully developed. We now 

 see the significance of the paratonic influence of light. The 

 growth of the stem is retarded, and, at the same time, the 



