22 EDITH SCHWARTZ CLEMENTS 



the greater number of monocotyledons. It seems probable that the 

 impulse to the various development of leaves comes from the vena- 

 tion of young leaves. If this stretches more decidedly in the shade 

 than in the sun, the leaf becomes larger and thinner, and its cells 

 drawn more to the surface, than in sun-leaves, where the cells, be- 

 cause of the lesser pull of the veins, stretch out more in the vertical 

 direction. 



Kriiger (83) investigated twenty-three species and eighteen 

 genera, noting the histological relations which stand in evident con- 

 nection with the conditions of life. He ascribed their manifold 

 structure to adaptation and heredity, but considered the influence of 

 the former alone, attempting to show, in a more or less general way, 

 how the structure of leaves, buds and stems corresponds to the 

 demands of the habitat. For instance, a certain group of orchids 

 becomes succulent in order to protect against transpiration, and to 

 store up necessary water. Another type produces water-storage 

 cells, and a thick cuticle to reduce transpiration, etc. 



Vesque (83) studied the histology of the leaves of the Caryo- 

 phyllaceae with regard to its bearing on their systematic arrange- 

 ment. 



Johow (84) made a large number of observations in the Lesser 

 Antilles on the relations of leaves to light. He did but little with 

 histology, but noted mainly variations in position and in movement 

 for protection against harmful illumination and excessive transpira- 

 tion, as well as protective modifications against too intense illumina- 

 tion. 



Heinricher (84) made an extensive study of the " isolateral " leaf 

 structure of a large number of plants from different parts of the 

 world. His conclusions are as follows : Isolateral leaf structure 

 is connected with the vertical position of leaves, and is due to the 

 effect of the sunny and, as a rule, dry situations of the species pos- 

 sessing it. Both factors, light and dryness, usually occur united, 

 but the latter does not seem necessary for isolateral structure; it is 

 secondary to strong illumination. Isolateral structure of leaves is 

 found more or less plainly in damp situations. Two species of 

 Boltonia possess it, the most hydrophytic one, however, only in- 

 completely. On the other hand, many swamp and water plants 

 with vertical leaves do not have isolateral structure, but do possess a 

 layer of palisade on the under side. High light intensities are re- 

 sponsible for isolateral structure, which has usually arisen from the 

 transformation of the sponge tissue of dorsiventral leaves into 



