82 



EDITH SCHWARTZ CLEMENTS 



LIGHT UNCHANGED; WATER AND HUMIDITY DECREASED 



The inference that a decrease in available water tends to a closer 

 arrangement of the chlorenchym cells is here confirmed by the fact 

 that three out of the four species have changed in this way. The 

 decrease in water for the group is accompanied by a decrease in 

 humidity in as much as the compared plant is in each case the 

 alpine form of the species. Since a decrease in humidity tends to 

 increase transpiration, and since the water-supply is reduced, adapta- 

 tions are in the direction of protection against water loss. Hence 

 intercellular spaces are reduced, and the cuticle may be thickened. 

 The further fact that three out of the four plants have relatively 

 thicker leaves (the dwarf form of Polygonum has a leaf as thick 

 but smaller than the type) indicates that dryness tends to increase 

 leaf thickness. 



LIGHT UNCHANGED ; WATER AND HUMIDITY INCREASED 



Conditions of moisture opposed to those for the preceding group 

 obtain for this one, and the converse effects on leaf structure rule. 

 In every case the leaf is thinner and the cells are more loosely 

 arranged. The tension in the leaf on account of surface growth 



