68 PRACTICAL BOTANY 



surface as possible to the water, in order that they may ab- 

 sorb from it the gases dissolved in it. Their shape somewhat 

 resembles that of the gills of fishes, and the thread-like divi- 

 sions of the leaf and the gill both have to do the work of 

 absorbing dissolved gases. 



62. Leaves in relation to water supply. The form and size 

 of leaves are frequently dependent on the water supply which 

 the plant receives. In many plants which grow in moist soil 

 or even in swamps the leaves are large and often entire, as 

 in the Cypripedium, skunk cabbage, white hellebore, papaw, 

 and the magnolias. 



In very dry soils or where the rainfall is scanty or lacking 

 during a considerable part of the warm months, there occur 

 many plants whose leaf surface is very small, as in some Eu- 

 phorbias, aloes, and heaths (Erica) ; or is even practically 

 wanting, as in most cacti (Fig. 65). This reduced leaf 

 surface evidently fits plants admirably to resist death from 

 excessive transpiration during droughts. 



When the soil temperature is nearly at the freezing point 

 most plants are unable to absorb much water by their roots. 

 It is probably mainly due to this fact that our ordinary winter 

 deciduous trees owe their habit of shedding the leaves at the 

 approach of winter. If then- actively transpiring leaves were 

 to remain at work while the ground was almost or quite frozen, 

 the tree would suffer a fatal loss of water. Winter deciduous- 

 ness is not a perfectly definite phenomenon, always setting in 

 at precisely the same season. For example, the common Jap- 

 anese honeysuckle, which is deciduous in the late autumn or 

 early winter in the Northeastern States, is almost or quite ever- 

 green in the South, and the trumpet honeysuckle is deciduous 

 in the North and perfectly evergreen in the South. Such de- 

 ciduous trees as the American tulip tree (Liriodendrori) and 

 the English oak become irregularly evergreen in the very 

 uniform climate of West Java ; that is, they show in December 

 and January (on separate boughs) a state of things correspond- 

 ing to their winter, spring, and summer condition in their 



