POWER OF MOVEMENT IN PLANTS. 307 



pendent of it. This — contrary to some German physiolo- 

 gists — we must conclude from both DeVries' and Darwin's 

 investigations. They are produced by the changing turges- 

 cence of the cells on different sides of a stem or footstalk, 

 which may or may not be fixed by some consequent growth 

 or solidification. This Mr. Darwin, we presume rightly, con- 

 cludes to be the faculty or susceptibility upon which heliotro- 

 pism, geotropism, and the like (not to speak of aphellotropism, 

 apogeotrojyism, paraheliotrojilsm^ cUaheliotropism, hi/ponasty, 

 ni/ctotrojnsm, and other terms which the incautious student 

 may take to be powers instead of abbreviated expressions) — 

 in other words, upon which the solar rays and some occidt in- 

 fluence of the earth — act, modifying the sweeps or converting 

 them into forth and back or other special movements. Among 

 these, that which has been termed the sleep of leaves, better 

 and briefly designated by the word nyctotropism, is thor- 

 oughly investigated in this volume, and is shown to be far 

 more general than has been supposed ; and the conclusion is 

 that the end subserved is a needful protection of the surfaces, 

 mainly of the superior surface against cold from nocturnal 

 radiation. A 2)riori, looking at the structure of the leaf, one 

 would have thought that the under surface had the greater 

 need of such protection. 



Not only are all these movements incipient in the seedling, 

 but some of them are manifested more rapidly and extensively 

 than in most mature plants. This should needs be, since, as 

 Mr. Darwin states it, " Seedlings are subjected to a severe 

 struggle for life, and it appears to be highly important to 

 them that they should adapt themselves as quickly and as 

 perfectly as possible to their conditions." Very properly, 

 therefore, no small part of this volume is devoted to the seed- 

 ling and to the behavior of its several parts. The most novel 

 and unexpected results relate to the young root. Judging 

 from its simplicity and from the medium in which it is de- 

 veloped, one would not look there for the endowment which 

 Mr. Darwin finds in it. But this root-tip and the vegetable 

 cells which compose it conspire to teach us that the most sim- 

 ple structures may be wonderfully gifted. The tiny root ex- 



