274 EVOLUTION OF PLANTS 



special tubers are formed, somewliat as in so many- 

 flowering plants. 



Sometimes instead of having the leaves much reduced 

 in size, the trees and shrubs of dry, hot regions may 

 have the position of the leaves such as to neutralize, to 

 some extent, the power of the sun's rays. Instead of 

 being placed horizontally, as most leaves are, in this 

 class of xerophytes the leaf hangs vertically and both 

 sides are alike. The various species of Eucalyptus, or 

 Australian gum trees, show this in a very perfect way, 

 and in western America there are a few examples, one 

 of the best being the manzanita (Arctostaphylos) of 

 the Californian mountains. 



Many tropical trees, whose leaves at maturity show 

 the normal position, have the young leaves pendent, so 

 that they are protected from the full force of the sun's 

 rays ; these are also very commonly colored pink or 

 crimson owing to the presence of red cell-sap in the 

 outer cells, and this probably serves as a screen to 

 protect the young chloroplasts. 



It is interesting to trace the development of some of 

 these modifications as they take place in the growth of 

 the young plant. Thus the seedling Eucalyptus has 

 broad, horizontal leaves, which also often occur in 

 young shoots of the older trees, and these are gradually 

 replaced by the pendent leaves with their vertically set 

 lamina. In many of the Australian Acacias, where 

 the lamina of the leaf is completely suppressed in the 

 older plant, and replaced by the vertically flattened leaf- 

 stalks, or phyllodia, the young plant has the feathery 

 pinnate leaves characteristic of so many Leguminosae, 

 and the transition from these to the phyllodia is very 



