94 THE BOOK OF NATURE STUDY 



facing the stem. The leaf blade consists of three lobes, two 

 attached opposite to one another by very short stalks, and the 

 third borne on a continuation of the leaf -stalk. Each leaflet 

 has a triangular outline widening out from the base, and is in 

 turn divided by deep indentations into three lobes, the margins 

 of which are toothed. The lower surface and margins of the 

 blade bear long and fairly stiff, white hairs. The branching of 

 the shoot proceeds from buds in the axils of the leaves, but as 

 it is seen only in the region bearing the flowers, or in the pro- 

 duction of creeping branches, its consideration may be deferred. 



The almost universal differentiation of the shoot in the higher 

 plants into a stem bearing thin flat leaves points to these parts 

 performing important functions different from those of the root 

 system. The leaves especially are constructed to play an im- 

 portant part in obtaining and preparing the food materials, at 

 the expense of which the plant grows. We have seen that the 

 roots absorb water, in which salts are dissolved, from the soil. 

 This passes up the stem, which thus serves to support and display 

 the leaves, and as the path by which the water reaches them. 

 The water passes up the leaf-stalk and is distributed by the 

 veins through the leaf -blade. The ordinary leaf -blade is charac- 

 teristically thin, and exposes a large surface to the air. One 

 use of this is to enable the plant to get rid of a great part of the 

 water absorbed by the roots ; this water escapes in the form 

 of vapour. The salts dissolved in it are left behind and accumulate 

 in the leaf, where they form part of the food material needed by 

 the plant. 



The expanded shape of the leaf, together with the green 

 colour found in most parts of the plant exposed to the light, 

 but especially in the leaf-blade, fits the leaves for their other 

 great use in the life of the plant. The plant takes up, mainly 

 by its leaves, a gas called carbonic acid gas, which exists in small 

 quantity in the air. From this gas, and not from the soil, the 

 carbon, which is one of the most important food materials of 

 the plant, is obtained. From the carbonic acid gas, the water 

 and the salts, which we have now followed to the leaf-blade, a 

 green plant can build up more complex substances and continue 

 to live and grow. The process of construction of these complex 



