RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 23 



_ palisade tissue, for the reason that isolateral leaves become more or 

 less dorsiventral when grown in the shade. Isolateral structure, 

 then, is ancestral in illuminated leaves and dorsiventral structure 

 is atavistic in shaded leaves. The stronger the light to which a plant 

 is exposed the more vigorously is the assimilative system developed, 

 and since for dicotyledons the typical assimilative cells are the 

 palisade cells, the entire mesophyll is often made up of palisade 

 cells. Light does not directly affect the form of the cells. It is the 

 factor which leads to greater assimilative activity, and this is best 

 carried out in the elongated form of the palisade cell. All progress 

 in the structure of the assimilative apparatus as well as its quantita- 

 tive formation is caused by light. Haberlandt's conclusion that the 

 position of the assimilative cells is dependent upon the transport of 

 food and not, as Pick thinks, upon light, is confirmed. The posi- 

 tion of the palisade cells is, as a rule, perpendicular to the surface, 

 whereas the leaf occupies all positions with reference to the direc- 

 tion of the light. The displacement of the palisade from the posi- 

 tion perpendicular to the surface, is due to the growth of the other 

 tissue elements. The assimilative cells bend towards the bundles. 

 This bending is not noticeable where a sponge tissue is differentiated 

 consisting of many armed cells which take up the function of trans- 

 port. Exceptions occur where there is no curving of the cells, and 

 where the cells of the bundle sheath have their long axes parallel to 

 the other cells, and perhaps even no connection at all between the 

 cells. This question is left for further investigation with the sug- 

 gestion of the possible development of the epidermis into a means of 

 transport. 



Costantin (85) studied the morphology and histology of the 

 leaves of a number of common aquatic plants. He noted the 

 acquisition or loss of certain characters as the leaves were sub- 

 merged or aerial, and also such adaptations as the presence or ab- 

 sence of stomata and hairs, and the amount of palisade tissue and 

 air-passages. He also gave a discussion of plant distribution and 

 adaptation to environment in general. 



Schenck (86) treated of aquatic plants in a thorough manner, 

 giving a detailed description and the life-history of a number of 

 species as well as their adaptations to the habitat, and the changes 

 which have taken place in the land forms of aquatic species. 



Mobius (87) made a comparative study of the structure of or- 

 chids, and, while admitting that climate and situation affect leaf 

 structure, referred to it only in single instances. He covered 193 



