i84 CONCLUDING KEMARKS AND Chap. IX 



Concluding Eemaeks and Sdmmaey of Chaptee. 



We do not know whether it is a general rule with 

 seedling plants that the illumination of the upper 

 part determines the curvatm-e of the lower part. But 

 as this occurred in the four species examined by us, 

 belonging to such distinct families as the Gramineee, 

 Cruciferse, and Chenopodese, it is probably of common 

 occurrence. It can hardly fail to be of service to seed- 

 lings, by aiding them to find the shortest path from 

 the biu'ied seed to the light, on nearly the same 

 principle that the eyes of most of the lower crawling 

 animals are seated at the anterior ends of their bodies. 

 It is extremely doubtful whether with fully developed 

 plants the illumination of one part ever affects the 

 curvature of another part. The summits of 5 young 

 plants of Asparagus officinalis (varying in height be- 

 tween 1"1 and 2'7 inches, and consisting of several 

 short internodes) were covered with caps of tin-foil 

 from 0'3 to 0"35 inch in depth; and the lower un- 

 covered parts became as much curved towards a lateral 

 light, as were the free seedlings in the same pots. 

 Other seedlings of the same plant had their summits 

 painted with Indian ink with the same negative result. 

 Pieces of blackened paper were gummed to the edges 

 and over the blades of some leaves on young plants of 

 Tropseolum majus and Ranunculus Jicaria ; these were 

 then placed in a box before a window, and the petioles 

 of the protected leaves became curved towards the 

 light, as much as those of the unprotected leaves. 



The foregoing cases with respect to seedling plants 

 have been fully described, not only because the trans- 

 mission of any effect from light is a new physiological 

 fact, but because we think it tends to modify somewhat 

 the current views on heliotropic movements. Until 



