66 A TEXTBOOK OF GENERAL BOTANY 
plants (Figs. 48, 511-513) ; low rates of evaporation and abun- 
dant moisture in the soil, other conditions being favorable, bring 
about a dense forest of tall trees (Figs. 6, 488). 
Regulation of transpiration. The rate of transpiration is regu- 
lated more or less temporarily in a number of different ways. 
The most important of these are by deciduous leaves, by drying 
of the walls of the chlorenchyma, and by opening and closing 

Fig. 48. Desert region in Arizona with Yucca alata to the left and 
Prosopis velutina to the right 
of stomata. There are also some other means, such as rolling 
and folding of leaves, which reduce the transpiring surface. 
Deciduous leaves. Broad-leaved trees which grow in regions 
that have either a very cold or a very dry season usually shed 
all their leaves during such periods of cold or of drought. Of 
course the shedding of the leaves greatly restricts transpiration. 
_ When the weather is very cold, plant roots can absorb little 
water from the soil, while if the leaves were exposed to the air, 
they would continue to transpire. This is one of the reasons why 
broad-leaved trees in the colder parts of the temperate zone shed 
their leaves at about the beginning of the cold season (Fig. 496). 
A very similar result ensues when trees are subjected to a 
Jong, pronounced dry season. At such times both the soil and 

