THE LEAF 67 
the air will usually contain comparatively little moisture, with 
the result that if trees had an extensive leaf area, their roots 
could not absorb enough water to supply that which would be 
lost by transpiration. The shedding of leaves by plants of the 
temperate zone during the au- 
tumn, and by tropical plants 
which grow in regions with 
a pronounced dry season, is 
therefore the result of very 
similar circumstances. In trop- 
ical regions where there is no 
pronounced dry season, plants 
retain their leaves throughout 
Fig. 49. Surface view of a stoma of the ae: 
Wandering Jew (Zebrina pendula). The drying of chlorenchyma 
(x 145) walls. On hot, dry, sunny 
days the leaves of plants may 
lose more water than the roots can absorb. The result of this 
is that the cells, and also the cell walls, then contain less water 
than when transpiration is less rapid. Very wet substances 
naturally lose more water 
than drier ones; and so, as’ 
the cell walls become drier, 
less water diffuses from 
them into the intercellular 
spaces. The rate of tran- 
spiration is thus automati- 
cally lowered. 
Opening and closing of sto-  ,,,,, 50. Cross section of a stoma and 
mata. Astomaisanopening surrounding cells of Zebrina pendula. 
between two sausage-shaped (x 130) 
cells called guard cells. Fig. 
49 shows a view of a partly open stoma, and Fig. 50 a cross 
section through a closed stoma and surrounding cells. The 
guard cells are constructed in such a way that they can move so 
as to open and close the stoma. Stomata are usually closed at 


