262 LECTURES TO SCIENCE TEACHERS. 



a more powerful action in retarding the growth of those 

 cells of the stem which are more immediately exposed to 

 them than they have in retarding that of the cells more 

 removed from their influence, and consequently the stem 

 becomes bent towards the source of light. 



To the phenomena resulting from the action of light 

 upon the process of growth the general term heliotropism 

 is applied. This young pea-plant, then, affords an example 

 of heliotropism, and since the direction of its curvature 

 is towards the source of light, the heliotropism is said 

 to be positive. It does happen, however, that in some 

 plants and parts of plants the direction of curvature 

 is away from the source of light (for instance, in the 

 tendrils of Vitis and Ampelopsis and many aerial roots), 

 and under these circumstances the heliotropism is said to 

 be negative. The explanation of this negative heliotropism 

 is by no means satisfactory as yet. Positive heliotropism 

 we said, depends upon a retardation of the growth of the 

 cells of the more brightly illuminated side of the stem, 

 and we might be inclined to go on to say that negative 

 heliotropism depends upon an increased rapidity of the 

 growth of the cells upon the more brightly illuminated 

 side to admit, in fact, that the cells of some plants have 

 their growth diminished by the action of light, whereas 

 the cells of others havs their growth increased. An 

 attempt has been made to explain the negative heliotropism 

 of tolerable transparent structures (e.g. the aerial roots of 

 Aroids) by showing that in such cases the side most 

 removed from the source of light receives a more intense 

 illumination than the side nearest to it in consequence of 

 the refractions which the rays of light undergo after their 

 penetration into the tissues of the organ (Wolkoff), and 

 under these circumstances, the apparent negative helio- 

 tropism of these organs would be merely a special case of 

 positive heliotropism. It is evident, however, that this 

 theory is quite inapplicable to the cases of negative 

 heliotropism occurring in organs which are not transparent. 

 The true explanation of negative heliotropism has yet, I 

 believe, to be discovered. For the present we may perhaps 

 be contented to assume provisionally the hypothesis that 

 growing cells are of two kinds, the cells of the one kind 

 having the rapidity of their growth increased, those of the 



