274 EVOLUTION OF PLANTS 
special tubers are formed, somewhat 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 
