_ 46 THE VEGETABLE WORLD. 
surrounded with a very remarkable cavity (alveola). The wood is 
_ traversed by resinife- 
rous ducts, which are 
a kind of interstice in 
which the resin pro- 
duced by the peculiar 
cells surrounding it is 
deposited and accumu- 
lates. Fig. 53 repre- 
sents the transverse 
section of the stem of 
Fir-tree. We see that 
evergreen trees, like 
forest trees, present 
a central medullary 
canal, concentri¢ 
woody layers, and 
cortical layers. But 
the wood-vessels have 
no existence in these 
stems, and the medul- 
lary rays are scarcely 
visible. The stem 
thus formed is said to 
beexogenous,fromebw, 
“without,” and yev- 
vaw, “I produce.” All @ 
indigenous trees, and 
a vast number of" our 
smaller shrubs and 
plants, have this form 
of stem. 
The generalappear- 
ance of palm-trees is 
very different from 
so 
7 
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drawn out stem (Lat. : 
