100 ANOMALOUS STRUCTURE OF MONOCOTYLEDONOUS STEMS. 
at the nodes, b, where the leaves arise, at which parts solid 
partitions are formed across the cavity, by which it is divided 
into a number of separate portions. Such stems when exa4 
mined at their first development present the usual endogenous 
structure, but in consequence of their growth in diameter taking 
place more rapidly than new matter can be deposited in their 
interior, the central tissue becomes ruptured, and they soon 
become hollow. 
In the stems of some other Monocotyledons we have a more 
striking deviation from the ordinary structure. Thus the 
species of Sarsaparilla and some allied plants have aerial stems 
which are strictly endogenous in structure, and underground 
stems which have the fibro-vascular bundles arranged in a ring 
Fre. 201. Fre. 202. 
> 
Fig. 201. Transverse section of the stem of the common Reed. a, Cavity 
closed at the bottom by a partition. 6. Annular scar indicating the 
point (node) where the leaf was attached——/ig. 202. Half of a trans- 
verse section of the underground stem of a species of Sarsaparilla. 
a. Epidermal tissue. 06, c, d. The cortical portion, e. Woody ring. 
jf. Central parenchyma. 
(fig. 202, e), around a central parenchyma, f, like the wood about 
the pith of an exogenous stem : such fibro-vascular bundles have, 
however, no cambium layer lke that which forms the rings of 
wood of an exogenous stem, and have consequently no power 
of indefinite increase like them. 
Age of Monocotyledonous Trees.—There is nothing in the 
internal structure of endogenous stems by which we can.ascer- 
tain the age of monocotyledonous trees as in those of exo- 
genous structure. It is supposed that the age of a Palm tree 
is indicated by the annular scars (jig. 199, 1) which are produced 
on the external surface of its stem by the fall of the terminal 
tufts of leaves; for, as one tuft only is commonly produced 
annually, each ring marks a year’s growth, and hence the number 
of annular scars corresponds to the number of years the tree 
has lived. Although it is true that in some few cases such a 
