20 EDITH SCHWARTZ CLEMENTS 



Three objects have been kept clearly in mind : ( i ) To correlate in 

 a definite way the histology of leaves with the measured physical 

 factors of their habitat; (2) to determine the kind and amount of 

 modification taking place in the leaves of the same species in dif- 

 ferent habitats; (3) to throw light upon the plasticity of different 

 species and genera. 



This investigation was suggested by Doctor Frederic E. Clements, 

 under whose direction it has been carried on. Grateful acknowl- 

 edgment is here made for helpful suggestions, and for the facilities 

 offered by the Department of Botany at the University of Nebraska, 

 and at the Alpine Laboratory, Minnehaha, Colorado. 



II. HISTORICAL 



Areschoug (78) studied the histology of about fifty plants from 

 the dicotyledons and monocotyledons, and some ferns, and concluded 

 that "the various anatomical types of the dicotyledons are par- 

 ticular forms of one and the same fundamental type which changes 

 in unlike conditions of life." 



Jonsson (80) investigated the histology of only a particular 

 family, the Proteaceae. 



Haberlandt (81) investigated the leaf histology of a large num- 

 ber of genera and species. He studied them especially with refer- 

 ence to the assimilative cells, and attempted to make clear the relation 

 in which the structure and arrangement of these cells stand to the 

 process of assimilation, and to prove by means of this relation that 

 the assimilative cells are physiologically uniform in spite of the 

 great variety of single constructions. He concluded from his in- 

 vestigations that in leaves of similar structure the specific assimila- 

 tive energy is approximately proportional to the entire mass of 

 chloroplasts in the leaf units concerned. He also noticed a peculiar 

 form of palisade cell which he called " arm-palisade," and explained 

 the form as due to infoldings of the cell wall for the purpose of in- 

 creasing the assimilative surface, since chloroplasts were noted in 

 great abundance next to the cell wall. He furthermore classified 

 leaves into types according to the structure of the mesophyll with 

 respect to the physiological principle of the most direct transport of 

 food materials. This principle was considered of more importance 

 than that of the direction of light, in determining the position of 

 palisade cells. The latter he concluded to be perpendicular to the 

 surface of the leaf as a rule, and not parallel to the rays of light. 

 He has explained intercellular spaces as serving, besides the purpose 



