RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 



This group of polydemics is similar to the preceding one but 

 with an increase in the available water. The, effect of this is im- 

 mediately seen in the looser arrangement and the increased oblate- 

 ness of the sponge tissue. The increase in thickness indicated by 

 some of the species must be viewed in the light of the fact that the 

 shade-leaves are very much larger than the corresponding sun- 

 leaves. This is especially striking in the case of Bursa bursa- 

 pastoris which has an extremely large thin leaf in the shade and a 

 very tiny thick one in the sun, and yet the absolute measurements 

 show a thicker leaf in the shade. In the case of the diplophyll 

 and staurophyll types, the sponge of the shade form has been evolved 

 from the lower prolate and oblobate cells. Another point brought 

 out by the above group is the relative stability of some species as 

 compared with others. This is generally true of the composites, 

 although they too show striking modifications in some cases. 



LIGHT INCREASED; WATER UNCHANGED; HUMIDITY DECREASED 



