322 



A TEXT-BOOK OF BOTANY. 



Epigeous Shoots. As would be supposed, these two kinds 

 of shoots vary to a certain extent. In epigeous shoots a number 

 of features may be noted. If the internodes are long the leaves 

 do not usually interfere with one another so far as exposure to 

 light is concerned, but if the internodes are short, the leaves are 

 all brought close together on the axis, and hence, were it not for 



B 



FIG. 1 80. A, woody vine of Canada moonseed (Menispermum canadense) , which ascends 



by twining to the right. 

 B, stem of wild yamroot (Dioscorea mllosa), which ascends by twining to the left 



and several of the characteristic 3-winged capsules at the top. 



The twining movements of stem climbers are due to the stimulus of gravity rathei 

 than to contact stimulus, and in the majority of twining plants the revolving movements, 

 as seen from the side, are from the left to the right, i.e., in a direction opposite to that of 

 the hands of a watch if represented diagrammatically. 



various modifications, their relation to light would be very un- 

 equal. Sometimes the shoot-axis may share with the leaves the 

 work of assimilation, as in the case of certain green stems. Then 

 again there are cases in which the leaves are reduced, and the 

 work of assimilation is carried on exclusively by the shoot-axis, 

 as in most Cactacese, certain marsh-plants, and others. On the 



