332 



EXPA'BIMEA^TS WITH PLANTS 



The discarding of leaves in winter by deciduous 

 trees and shrubs is an adaptation to the drought from 

 which the plant suffers in winter, since the roots are 

 then unable to absorb sufficient 

 water to supply the leaves. We 

 may test this by the following 

 experiment : Take two vigorous 

 plants growing in pots ; place one 

 on ice (by means of the arrange- 

 ment shown in Fig. 11) ; keep the 

 other near it at normal tempera- 

 ture; give both the usual quantity 

 of water. Which plant wilts first? 

 On continuing the experiment 

 for a few days, it will be found 

 that the plant on ice sheds its 

 leaves (Begonia may be recom- 

 mended for this experiment) . 



The fall of the leaf is preceded by the withdrawal 

 of all nutriment from it and the formation of a layer 

 of loose cells at the base of the stalk ; this layer 

 finally breaks apart, heaving a clear, smooth scar 

 (see page 212). The disorganization of the chloro- 

 phyll gives rise to the gorgeous colors of autumnal 

 foliage and, while not caused Ijy frost, is hastened 

 by it. 



Trees which retain their leaves in winter have 

 various devices to restrict evaporation. Study in this 



183. Branch of Cytisus (a switch 

 plant), showing the green 

 switch-like branches which 

 perform the work of leaves. 



