RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 



Si 



Light 1/50; water +(9.5-12.5%); humidity +(5-10%); tem- 

 perature (3_4). 



BOG : light I ; available water 58% ; humidity 50-70% ; tempera- 

 ture 50-75 : the leaf is like that of the shade form with a thin 

 cuticle. 



Light i ; water +(5 2 -5-55-5%) J humidity +(5-20%) ; tempera- 

 ture the same. 



VI. GROUPING OF POLYDEMIC SPECIES 



In the following tables the habitat forms of polydemic species 

 are grouped according to the physical factor differences between 

 their habitats and that of the type form. Structural differences in 

 the leaves of the plants themselves are expressed as briefly and as 

 graphically as possible. 



LIGHT AND HUMIDITY UNCHANGED ; WATER DECREASED 



Because of the small number of species subjected to the influence 

 of a decrease in available water alone, no general inferences can be 

 drawn. Mertensia polyphylla indicates that the effect of decreased 

 water is to be found in the arrangement of the cells alone. The 

 closer position of both the palisade and sponge cells decreases air- 

 spaces, and, in consequence, loss of water by transpiration. In the 

 case of Senecio rosulatus the water-supply of the form growing 

 on the rock is at a minimum. The available water for the type 

 form is already small, and the only adaptation to a decrease is a 

 dwarfing of the entire plant. 



