io8 



BOTANY AND PHARMACOGNOSY. 



Usually there is a more compact arrangement or stronger develop- 

 ment of chlorophyll tissue on the upper or ventral surface, while 

 on the lower or dorsal surface the veins stand out more promi- 

 nently, and there is a greater number of stomata or breathing 

 pores. Frequently when the wind is blowing strongly the dorsal 

 surface of the leaves will be brought into view and in some 

 instances they appear almost white, as in certain maples and 

 willows. 



In contrast with this type of leaf may be mentioned those 

 which grow edgewise and in which both surfaces of the leaf are 

 more or less alike, as in the Eucalypts and Acacias of Australia. 



Fig. 71. American senna (Cassia Tnarilandica). The figure at the left shows the pin- 

 nately-compound leaves in the day position when under the influence of light, and the one 

 to the right the drooping position of the leaflets at night. 



Such leaves are bilateral, and other examples are furnished by 

 Iris, Cat-tail (Typha) and Calamus. Another type of leaf is the 

 radial. This is illustrated by Juncus, the leaves of which are 

 hollow and cylindric. 



Functions of the Leaf. — When we speak of the leaves of 

 the plant we usually have in mjnd the foliage leaves or green 

 chlorophyll leaves. 



Under the influence of sunlight the chloroplasts are able to 

 rearrange the elements in carbon dioxide and water, which are 

 looked upon as inorganic substances, into starch or related com- 



