RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 



is indicated in the tendency of the sponge cells to take the oblate 

 shape. These changes must be referred directly to the influence of 

 an increase in humidity and available water, since the light remains 

 the same. They are in the direction of increase in transpiring sur- 

 face. In Chamaenerium the palisade even is influenced in this 

 direction, since the second row is becoming sponge-like. 



LIGHT DECREASED; WATER UNCHANGED; HUMIDITY INCREASED 



The conditions for the above group are those of deeper shade, 

 the forms being either shade-forms as compared with sun-forms, 

 or forms growing in differing degrees of shade. In the latter case, 

 the water and humidity for some of the species have not been 

 measured, but the humidity at least is greater the deeper the shade. 

 For these it is possible that the soil-water is somewhat greater as 

 well. The important factor, however, is the decreased light and to 

 it can be referred the main changes in the structure of the leaf. 



