HELIOTROPISM. 679 



Perhaps these phenomena of growth-retardation in light are 

 partly the result of dispersion of energy. In light there is helio- 

 tropism with other forms of irritability, chlorophyl-making, as- 

 similation, and growth ; while in darkness there is hut response 

 to the influence of gravity and growth. Buds which contain a 

 definite amount of stored-up energy will, perhajDS, bring to pass 

 diiferent results, as the number of uses made of this energy may 

 vary. Nature is constantly performing experiments along these 

 lines which indicate such a probability. The alternation of day 

 and night presents a natural periodic etiolation of most normally 

 situated plants. Examination will show that growth is more rapid 

 in darkness than in light, and in many plants it is only at night 

 that any considerable increase of size takes place. Some plants, 

 like the hop — which chances to be apheliotropic — do not grow 

 more rapidly in darkness, and this may be attributed to a differ- 

 ence in irritability, or perhaps an inhibition of the growth process 

 ' by some other. At night, however, the temperature is lower 

 than in the daytime, and this is inimical to growth. Again, some 

 flowers open only at night, and since the opening of a flower is 

 evidence that growth is retarded, another apparently abnormal 

 case is offered. Such instances are unusual and poorly under- 

 stood. In general, darkness seems to favor a maximum of tur- 

 gescence in plant-cells. 



Motion, now, in plants is a phenomenon of growth — not, very 

 possibly, of growth, viewing the plant as a whole, but considering 

 the cells separately. The sunflower turns to the sun because 

 upon the side next the source of illumination, the cells possess a 

 kind of irritability, in view of which, through molecular changes 

 in the irritated protox^lasm — making it more absorbent — growth 

 is retarded and curvature ensues. The old theory of De CandoUe 

 differed from this in that light was supposed to be inimical to nu- 

 trition, or cell-formation, and the meaning of turgescence and of 

 irritability was not clearly understood. The ivy, which turns 

 away from the sun, may possibly be accredited with a different 

 kind of irritability, or, what seems more reasonable, habit may 

 act as an inhibitor. 



"With regard to simple cells, the terms of the law that helio- 

 tropism is a phenomenon of growth must be modified a little. 

 The Bacterium plwtometricum of Engelmann, which moves only 

 under the influence of light, does not at the same time increase in 

 size. Neither do the filaments of the Oscillaria nor the zoospores 

 of algse. The plasmodia of slime-molds, which, except during 

 the spore-forming stage, are negatively heliotropic, do not grow 

 while turning from the light. Irritability in these organisms is 

 translated at once into ciliary or mass movement, and has to do 

 with growth only in a secondary way. It is, however, clearly 



