INTERNAL STRUCTURE OE STEMS. 
41 
109. BlllbletS are little bulbs, or fleshy buds, formed in the axils of leaves above 
ground, as in the Bulb-bearing Lily. Or in some Leeks and Onions they take the 
place of flower-buds. Falling off, they take root and grow into new plants. 
110. The Internal Structure of Stems. Plants are composed of two kinds of ma- 
terial, namely, Cellular Tissue and Wood. The former makes the softer, fleshy, and 
pithy parts ; the latter forms the harder, fibrous, or woody parts. The stems of 
herbs contain little Avood, and much cellular tissue ; those of shrubs and trees 
abound in the Avoody part. 
111. There are tAvo great classes of stems, which differ in the way the Avoody 
part is arranged in the cellular tissue. They are named the Exogenous, and the 
Endogenous. 
112. For examples of the first class we may take a Bean-stalk, a stem of Flax, 
SunfloAver, or the like, among herbs, and for Avoody stems any common stick 
of AA'ood. For examples of the second class take an Asparagus-shoot or a Corn- 
stalk, and in trees a Palm-stem. These names express 
the different Avays in Avliich the two kinds grow in thickness 
when they live -more than one year. But the difference 
betAveen the tAvo is almost as apparent the first year, and 
in the stems of herbs, Avliich last only one year. 
113. The Endogenous Stem. Endogenous means “inside- 
groAving.” Fig. 77 shows an Endogenous stem in a Corn- 
stalk, both in a cross-section, at the top, and also split 
doAAm lengtliAvise. The peculiarity is that the Avood is all 
in separate threads or bundles of fibres running lengthwise, 
and scattered among the cellular tissue throughout the 
AAdiole thickness of the stem. On the cross-section their 
cut ends appear as so many dots ; in the slice lengthwise 
they sIioav themselves to be threads or fibres of Avood. 
Fig. 73 is a similar Adeiv of a Palm-stem (namely, of our 
Carolina Palmetto, of Avhich AAdiole trees are represented 
in Fig. 79). It slaws the endogenous plan in a stem 
several years old. Here the bundles of Avood are merely 
increased very much in number, neAV threads having been 
formed throughout intermixed with the old, and any in- 
crease in diameter that has taken place is from a general distention or enlargement 
