EDITH SCHWARTZ CLEMENTS 



LIGHT INCREASED; WATER UNCHANGED; HUMIDITY DECREASED Continued 



Under the influence of increased light and the usual accompany- 

 ing reduction in humidity and available water, changes in leaf 

 structure opposite to those in the preceding group are to be ex- 

 pected and do obtain. The leaf is -increased in thickness, involving, 

 as a rule, increase in both palisade and sponge tissues. In the case 

 of Batrachium aquatile, the plant is dwarfed and the measured de- 

 crease is not relative to leaf surface. The leaf of Chamaenerium 

 is but a trifle thinner than for the type, although the light is slightly 

 greater. The cuticle is thickened as a rule, and the tendency of 

 both palisade and sponge cells is to extend vertically at the expense 

 of the horizontal axis. This form of cell is best adapted for screen- 

 ing the chlorophyll from over-illumination, and for most active 

 assimilation. In respect to arrangement of cells, there is a marked 

 reduction in intercellular spaces, thus decreasing transpiration and 

 interior illumination. The extreme adaptations for the group are 

 in the spongophyll type of aquatic leaves which have changed from 

 well aerated sponge tissue to characteristic prolate palisade cells 

 with few intercellular spaces. The dwarfing of these three species 

 may be referred to the great reduction in available water: in the 

 aquatic plant, absorption takes place by means of the entire plant, 

 whereas the amphibious form has but few tiny rootlets. 



