RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 45 



group is that alpine and foothill plants are so frequently grouped 

 under the same types. Such plants are exposed to similar amounts 

 of soil-water and of light as well as to low humidities, the latter 

 however from a different cause for each situation alpine humidity 

 being low because of decreased air-pressure, and that of the foot- 

 hills on account of the heat and radiation. Temperature is the only 

 variable factor, being high for the foothills and low for the alpine 

 region. This indicates that temperature is of less importance than 

 other physical factors in determining leaf structure. 



By far the larger number of composites are found among the 

 xerophytic types. The Antennaria type is narrower leaved than the 

 majority of xerophytes, and has but little palisade tissue. This is 

 due to the thick covering of hairs on either surface. It has been 

 found that such a covering acts as a screen to light, and that the 

 chlorenchym beneath has the structure typical of a shade-leaf. In 

 some cases the palisade is very loose under such conditions, espe- 

 cially where the leaf is rather thick, as in Rydbergia grandiflora. 

 This is also evident in Aragalus lamberti and Macronema pyg- 

 maeum, which vary from the type in possessing a looser arrange- 

 ment of the chlorenchym cells, and a woolly coating of hair. In 

 other cases, as in Lesquerella, a dense covering of stellate hairs by 

 reducing transpiration permits thinness in the cuticle. 



V. DISCUSSION OF POLYDEMIC SPECIES 



The polydemic species have been grouped into hydrophytes, 

 mesophytes and xerophytes according to the normal habitat of the 

 species considered. The form occupying the normal habitat is 

 taken as the type, and is described in connection with its physical 

 factors. For the other forms only the points of difference between 

 them and the type, either in physical factors or in structure, are 

 noted. Following each form is a summary of the differences of 

 both factors and structure with reference to the type. Abbrevia- 

 tions for this have been used as follows : " C," Chlorenchym ; " P," 

 Palisade ; " S," Sponge ; the plus and minus signs have been used 

 to show increase or decrease with reference to the type, upon which 

 the percentages are also based. 



