24 EDITH SCHWARTZ CLEMENTS 



species and 95 genera, giving histological descriptions and their 

 bearing on the systematic arrangement of the orchids. He was 

 especially interested in the influence of heredity, *. e., " the relations 

 between the similarity and dissimilarity of leaves in their more de- 

 tailed structure to the greater or less relationship in which the 

 species considered stand to one another." He nevertheless ad- 

 mitted that species systematically close together will be unlike in 

 histological structure if they occupy different habitats. 



Dufour (87) experimented with plants to find out the effects of 

 light, by growing different individuals of the same species in full 

 sunlight and in shade, keeping all other factors the same. His 

 results were uniformly in favor of greater development of the sun 

 plant in stem, leaves and roots. The leaves of the sun-plant com- 

 pared with those of the shade-plant were larger, thicker, with thicker 

 cuticle; palisade tissue was more highly developed as well as the 

 conductive and supportive tissues, and chloroplasts, starch and 

 crystals occurred in greater abundance. He ascribed the fact that 

 most investigators have described shade-leaves as usually larger 

 than sun-leaves to be due to the influence of water-content. To 

 prove this he grew plants in very wet and in very dry soil, the 

 other factors remaining the same, and found that the leaves of the 

 former were larger than those of the latter. He explained the oc- 

 currence in nature of larger leaves on shade-plants as being due 

 to the fact that as a rule a sunny place is dry and a shady one 

 moist. The preponderance of leaf surface then, would be with the 

 one or the other according to the resultant of the two inverse forces 

 in either spot. 



Loebel (89) discussed the structure of the leaves of a number 

 of plants, and their physiological importance, agreeing that the con- 

 ditions of life exercise a certain influence upon the inner organiza- 

 tion of leaves. 



Bonnier (90) established stations at different altitudes in the 

 Alps and on the plains in which he planted different parts or seeds 

 of the same plant, taking care that the soil from the higher regions 

 was taken to the plains so that the different plants grew in the same 

 soil. He also chose species which were not peculiar to either plains 

 or high altitudes, but rather those growing at an intermediate alti- 

 tude, that there might be no question of abnormal development. 

 Out of 203 roots planted 123 survived, and of these 119 remained 

 in the lower stations in such fashion that a comparison could be 

 established for almost all the species chosen. The plants were 



