io8 BOTANY AND PHARMACOGNOSY. 



incident rays of daylight, and not to that of the rays of the 

 morning sun. This phenomenon may be studied in the house 

 geranium and other window plants. In endeavoring to explain this 

 behavior of the leaves, Frank assumes it to be due to a kind of 

 heliotropic irritability peculiar to dorsiventral organs, and terms 



it TRANSVERSE HELIOTROPISM. 



The stem, as well as the petiole or stalk of the leaf, is also 

 influenced by the light, and is said to manifest positive helio- 

 tropism. Those parts of plants that turn away from the light, as 

 the aerial roots of the ivy, are said to possess negative helio- 

 tropism. 



Depending upon their relation to external agents, several forms 

 of leaves are distinguished. In those which assume a more or 

 less horizontal position the two surfaces of the lamina are quite 

 different, and the leaves are said to be dorsiventral, or bifacial. 

 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. 



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. 

 In Iris and Calamus, the leaf-like organ is actually not the blade, 

 but merely a part of the dorsal face, which, in the bud, has already 

 pushed out so as to exceed the apex. Such leaves are called 

 sword-shaped and are frequently referred to as eouitant. The 

 leaves of certain species of Juncus, Carex and some of the grasses 

 are commonly spoken of as cylindric. Such leaves are, how- 

 ever, only apparently cylindrical, since the ventral surface is 

 often distinct, though much narrower than the dorsal. They 

 are also frequently hollow. 



Functions of the Leaf, When we speak of the leaves of 

 the plant we usually have in mind 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- 



