THE LEAF | 65 
Fig. 47, which represents a cross section of a leaf, shows an 
interesting hereditary modification suited to a particular kind 
of environment. The thick, water-storing epidermis helps the 
plant to endure short periods of comparative dryness, while the 
single layer of chloroplastids indicates a shady habitat. 
Effect of evaporation on distribution of plants. In nature we 
find that plants of desert, fairly moist, and very moist regions 
are all specially fitted for the con- 
ditions under which they grow, and oa ae 
that the structural modifications 
which fit them for their native local- 
ities are fixed by heredity to such 
an extent that usually they will not 
thrive under conditions that are very 
different from those of their natural 
habitat. The rate of evaporation 
is one of the most important envi- 
ronmental factors for which plants 
are fitted, and the rate of evapora- 
tion determines, to a considerable 
extent, the kinds of plants which 
grow naturally or can be cultivated Fia.47. Cross section of a leaf 
profitably on any given area. If the of Peperomia pellucida, a trop- 
soil is sufficiently irrigated, many ical plant that thrives on shady 
crop and garden plants which are song RA 
suited to fairly moist conditions can pie. Watersstar SAU Dees bes 
y ; ermis and single layer of cells 
be grown successfully in regions that with chloroplastids 
are naturally dry; but there are many 
plants which cannot withstand the very high rates of evaporation 
prevailing in arid regions, no matter how much water is in the soil. 
Many desert plants can stand much more moist conditions than 
those under which they normally occur, and can be successfully 
cultivated in moist regions; but when left to themselves they are 
not able to compete with the vigorous plants of moist areas. 
High rates of evaporation, accompanied by a small percent- 
age of water in the soil, result in an open growth of small 

