MORPHOLOGY OF HIGHER PLANTS. 349 



Light Relation of Leaves.— While the lamina of llie leaf 

 appears to assume a more or less horizontal position, it usually 

 inclines at such an angle as to receive the greatest amount of dif- 

 fused daylight. Wiesner has shown, for instance, that when 

 plants are so situated that they receive direct sunlight only for a 

 time in the morning, and diffused daylight during the rest of the 

 day, the position of the upper surface is at right angles to the 

 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 dorsivextral. or 

 bifacial. Usually there is a more compact arrangement or stronger 

 development of chlorophyll tissue on the upper or ventral surface, 

 while on the lower or dorsal surface the veins stand out more 

 prominently, 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. 



