EXTERNAL CONDITIONS ON PLANT LIFE. 



173 



right toward the western sun which is still quite high. At 

 sundown, after the influence of the falling dew has been slightly 

 felt, the leaves will begin to assume the position seen in 

 Plate No. 4. 



If we continue to watch this plant as darkness comes on, 

 we shall find all these leaflets dropping down slightly, each 

 turning itself one-fourth around on its petiole until the three 



present their edges up- ^ 



permost. The two outer 

 leaflets, having their faces 

 toward each other, move 

 slightly toward the cen- 

 tral one until they touch 

 it. It will be seen that 

 the three little leaflets 

 have thus placed them- 

 selves together in such a 

 way as to reduce their 

 surface to nearly one- 

 third of the original area. 

 This protects them from 

 radiation and the loss of 

 much heat during the 

 night. Plate No. 9 shows 

 a plant while sleeping, 

 the photograph having 

 been taken at twelve 

 o'clock midnight. 



But there are other 

 effects of light more 

 marked than these re- 

 actionary movements. In ordinary plant assimilation, which 

 takes place only under the influence of light, the leaves are 

 generally the active organs. It sometimes happens that when 

 the light becomes insufficient for assimilative purposes, certain 

 parts of the organ itself the leaf become quickly atrophied 

 or disappear entirely. Thus the form of the elm leaf has 

 gradually become oblique on account of the shading of one 



PLATE No. 9. 



