The Water Relations of Leaves 



57 



water needed for their 

 high rate of transpira- 

 tion. There are plants 

 like the dandelion that 

 can adjust themselves 

 to both these condi- 

 tions. Most plants, 

 however, cannot do this, 

 and those with a high 

 transpiration rate die 

 off on a dry hillside, 

 while those with a low 

 transpiration rate sur- 

 vive. This indicates 

 only one of the factors 

 which must be taken 

 into account in the selec- 

 tion Of plants for par- FlG " 37 ' Submerged plants. From left to right: 

 . eelgrass (Vallisneria), naiad (Najas), water weed 



ticular habitats ; Other (Elodea), and pond weed (Potamogeton). 



factors will be con- 

 sidered in connection with the study of stems and roots. 



Submerged and floating leaves. An examination of a 

 submerged leaf of any pondweed shows that it has no stomatal 

 openings. The floating leaves of water lilies and other pond 

 plants have stomata only on the upper surfaces. Evidently 

 submerged plants have no transpiration. It is also certain 

 that they get their carbon dioxid directly from the water 

 through the epidermis, for carbon dioxid is found dissolved in 

 pond waters, often in larger proportion than in the air. 



In water-lily leaves the upper surface is covered by a 

 cuticle that is not readily made wet, and it has stomata that 



