64 A TEXTBOOK OF GENERAL. BOTANY 
close together, they will have a tendency to decrease the rate 
of transpiration, as they tend to minimize the effect of the 
movement of air near the stomata. 
Hereditary and acquired modifications of structure. ‘The changes 
in structure which restrict or accelerate the rate of transpiration 
are of two general classes: those changes which are characteris- 
tic of a species and are hereditary, and those which are acquired 
by individual plants or leaves in response to the condition under 
which they are grown. 
Acquired modifications. Even on the same plant some leaves 
may be more exposed than others to the effects of evaporation, as 
they are not all equally exposed to the action of sun and wind. 
We usually find that on the same plant the leaves that grow 
in the sun are thicker and have more compact chlorenchyma 
and thicker epidermal walls than those that grow in the shade 
(Fig. 35). These differences are due to the response of the in- 
dividual leaves to their environment. A practical application is 
made. of this fact in growing tobacco under shade in order to 
obtain large, thin, and pliable leaves to be used as cigar wrap- 
pers. While leaves respond, to a certain degree, in the ways 
mentioned above, the possible extent of such response is limited. 
Hereditary modifications. Besides the response of individual 
leaves and plants to different environments there are hereditary 
modifications of leaf structure which may be more pronounced 
and which fit plants for different environmental conditions. On 
many desert plants the leaves are reduced to rudimentary struc- 
tures or are modified into spines (Figs. 43, 44, 512). No matter 
under what conditions these plants may be grown, they will not 
develop large leaves. Likewise, delicate ferns will not develop 
leaves which can resist the high rates of evaporation that occur 
in desert regions. Plants which are natives of the desert and 
have very small leaves may, if grown in moist regions, develop 
somewhat larger leaves, but they will never develop leaves of 
any great size. While environment may have a marked influ- 
ence on leaves, heredity is in general more important than 
environment in determining their size. 
