154 
A TEXTBOOK OF GENERAL BOTANY 
structure of coniferous leaves (Fig. 475) probably have some 
connection with the absence of vessels in coniferous stems. 

Frag. 130. Cross section of a small portion 
of the wood of a redwood tree, showing 
xylem on both sides of a pith ray 
The small cells in the lower part of the 
drawing are the last of the wood cells formed 
during a summer, while the large cells above 
them are the first xylem cells formed during 
the next spring. The fact that much larger 
cells are formed in the spring than later in 
the summer results in the marking of the 
wood by annual rings of growth. (x 275) 
bly connected with the ability of 
Most coniferous trees 
(Figs. 477, 478, 498) re- 
tain their leaves for sev- 
eral years, — in temperate 
countries throughout the 
long, cold winter, when the 
dicotyledonous trees lose 
their leaves. 
cussion of leaves it was 
pointed out that the loss of 
leaves by deciduous trees 
is a method of restricting 
the rate of transpiration at 
times when the roots are 
not able to absorb enough 
water to replace that which 
would be lost by the fo- 
lage. The deciduous tree 
without its leaves is one 
of the most perfectly pro- 
tected of plant structures, 
since all the exposed por- 
tions are effectively coy- 
ered by the impervious bud 
scales and bark. The low 
rate of transpiration from 
coniferous leaves is proba- 
these trees to retain their 
foliage throughout the cold winter. At such times coniferous 
trees have some advantage over dicotyledonous ones in that 
they can carry on photosynthesis. 
very rarely occur at low elevations, while they flourish in 
In the tropics conifers 
high mountainous regions where the rate of evaporation is 
always low. 


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