623 LECTURE XXXV. 



facts already contained in what has preceded. In the first place that in all the 

 phenomena here described it can only be a matter of the direction of the rays 

 of light. No one will assume that a swarm-spore swimming towards the light 

 does this because its anterior end is more strongly illuminated than its posterior. 

 Still less will it be supposed in the case of the chlorophyll-plate of Mesocarpus, 

 when assuming its position in plan or in profile, or in the corresponding move- 

 ments of chlorophyll-corpuscles, that the movements of the protoplasm referred to 

 can in any way be produced by the one side being more strongly illuminated 

 than the other ; on the contrary, it can only be a matter of the direction in which 

 the ray of light falls upon the irritable protoplasm. 



In the second place I would lay stress on the fact that both in the case 

 of swarm-spores and in that of the movements of protoplasm containing chloro- 

 phyll, entirely different, or even opposite effects occur, according as the incident 

 hght is feeble or very intense. This phenomenon also we shall again meet 

 with later in the heliotropism of shoot-axes and leaves. We shall see that 

 many such organs become curved concave on the illuminated side in a feeble 

 light, and in a strong light convex. 



I shall return again to these facts when treating of heliotropism. 



