THE STEM 93 
cramped to the outside of the stem to form the so-called 
inner bark. The corky outer bark, which, as it were, 
replaces and takes up the work of the epidermis, is 
formed from the outside layer of the cortex. On 
removing the inner bark, the white, glistening, juicy 
layer of the cambium may be seen. If, in a growing 
tree, a thin piece of metal be slipped between the 
cambium and the wood it will in the course of a year 
be found to be covered with a layer of wood which the 
cambium has produced. 
Now cut sections from the stem of a mono- 
cotyledon (Fig. 60). The flower stalk of a lily, being 
quite, typical, is convenient for the purpose. As in the 
cdicotyledon, there are epidermis and cortex, of which 
the former may again be skinned off. The cortex 
contains a considerable amount of chlorophyll and is 
therefore concerned in nutrition. At this point the 
resemblanee between monocotyledon and dicotyledon 
ceases. Inside the cortex, the former shows a well- 
marked pericycle, consisting of a dense ring of strong 
fibres which constitute the strengthening and _ sup- 
porting tissue of the plant. Enclosed by the pericycle 
and sometimes encroaching upon it, are the vascular 
bundles set in a colourless ground tissue not unlike the 
pith of a dicotyledon. The bundles are not arranged in 
a ring, but scattered throughout the ground tissue. 
No eambium is present, so that, when once the cells 
and vessels that constitute the bundles have reached 
their full size, there is no chance of further enlargement 
of the bundle itself. 
TYPES OF STEMS. 
Scientifically, the best classification of stems is, 
according to structure, into dicotyledons and mono- 
cotyledons, but, unfortunately, if we pursue this line 
we shall be forced to consider numerous microscopic 
differences that are beyond our scope. To classify 
