42 HOW PLANTS GROW. 
of the whole. Such stems may well enough be called inside-growers, because their 
wood increases in amount, as they grow older, by the formation of new threads or 
fibres of wood within or among the old. 
114. Moreover, endogenous stems 
are apt to make few or no branch- 
es. Asparagus is the only common 
example to the contrary; that 
branches freely. But the stalks 
of Corn and other grain, and those 
of Lilies (Fig. 1, 2) and the like, 
seldom branch until they come to 
flower ; and Palms are trees of 
this sort, with perfectly simple or 
branchless trunks, rising like col- 
umns, and crowned with a tuft of 
conspicuous and peculiar foliage, 
which all comes from the continued 
growth of a terminal bud. 
115. The Exogenous Stem is the 
kind we are familiar with in ordi-— 
nary wood. But it may be observed 
in the greater part of our herbs as 
well. It differs from the 
other class, even at the be- 
ginning, by the wood all 
occupying a certain part of 
the stem, and by its woody 
bundles soon appearing to 
run together into a solid 
layer. This layer of wood, 
whether much or little, is always situated around a central part, or pith, which 
.has no wood in it, being pure cellular tissue, and is itself surrounded by a bark 
which is mainly or at first entirely cellular tissue. So that a slice across an exoge- 
nous stem always has a separate cellular part, as bark, on the circumference, then a 
ring of wood, and in the centre a pith; as is seen in Fig. 80, representing a piece 
y 
Palmettus of various ages, and a Yucca, y. 
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