MORPHOLOGY OF ANGIOSPERMS. 109 



pounds which are of an organic nature. This process is known 

 as carbon dioxide assimilation, or photosynthesis, which latter 

 term means the building up of a compound under the influence of 

 light. In this process, which is sometimes expressed by the fol- 

 lowing formula, oxygen is given off : 



6CO, + 5H,0 = QH,„0, + 60, 



Carbon Dioxide Water Starch Oxygen 



The importance of this function can be best appreciated by 

 bearing in mind that all of the organic products built up by the 

 plant are derived almost entirely from the carbon dioxide of the 

 air which is taken in through the leaves. 



Transpiration and respiration are also functions of the leaf. 

 Transpiration is the giving off of water (through water-pores), 

 or watery vapor (through the stomata), which has been absorbed 

 by the root hairs and transported through the tissues of the root, 

 stem and leaf; the process of breathing, or respiration, consists 

 in the taking in of oxygen and giving off of carbon dioxide, the 

 exchange being just the reverse of what it is in photosynthesis. 

 These several functions are, however, not confined to the leaf 

 alone, but are carried on by all the green parts of the plant. 



Leaf Venation. — The foliage leaves of higher plants are 

 traversed by vascular bundles, which enter the blade through the 

 petiole and diverge at the base, or, as in the case of Dicotyledons, 

 branch in various ways ; and it will be seen that the form of the 

 leaves corresponds to the distribution of the bundles. These 

 bundles are known as veins or nerves, and they have two func- 

 tions, namely, (i) that of a mechanical support, and (2) that of 

 carrying nutritive materials to and from the leaves. 



The mode of venation in Monocotyledons and Dicotyledons 

 differs somewhat, but it will be found that in a number of instances 

 the venation of leaves of plants belonging to one of these great 

 groups will resemble that of the leaves of certain plants in the 

 other group. However, there are certain general types belonging 

 to each group. 



Venation in Monocotyledons. — An examination of the 

 leaf of lily-of-the-valley shows that the veins arise at the base 

 and run more or less parallel to the apex. Such a leaf is said to 

 be PARALLEL-VEINED or NERVED. It Will morcover be noticed that 



