CHAP. VII. SUMMARY ON SLEEP OF LEAVES. 409 



mobility of the stems and leaves in this family, as 

 shown by the large number of climbing species which 

 it contains. Next to the Leguminosae come the Mal- 

 vaceae, together with some closely allied families. But 

 by far the most important point in the list, is that we 

 meet with sleeping plants in 28 families, in all the 

 great divisions of the Phanerogamic series, and in one 

 Cryptogam. Now, although it is probable that with 

 the Leguminosoe the tendency to sleep may have been 

 inherited from one or a few progenitors, and possibly 

 so in the cohorts of the Malvales and Chenopodiales, 

 yet it is manifest that the tendency must have been 

 acquired by the several genera in the other families, 

 quite independently of one another. Hence the ques- 

 tion naturally arises, how has this been possible ? 

 and the answer, we cannot doubt, is that leaves owe 

 their nyctitropic movements to their habit of cir- 

 cumnutating, a habit common to all plants, and 

 everywhere ready for any beneficial development or 

 modification. 



It has been shown in the previous chapters that the 

 leaves and cotyledons of all plants are continually 

 moving up and down, generally to a slight but some- 

 times to a considerable extent, and that they describe 

 either one or several ellipses in the course of twenty- 

 four hours ; they are also so far affected by the alter- 

 nations of day and night that they generally, or 

 at least often, move periodically to a small extent; 

 and here we have a basis for the development of the 

 greater nyctitropic movements. That the movements 

 of leaves and cotyledons which do not sleep come 

 within the class of circumnutating movements cannot 

 be doubted, for they are closely similar to those of 

 hypocotyls, epicotyls, the stems of mature plants, and 

 of various other organs. Now, if we take the simplest 



