INTERNAL STRUCTURE OF STEMS. 



41 



109. BulblelS are little bulbs, or fleshy buds, formed in the axils of leaves above 

 ground, as in the Bulb-beariug Lily. Or in some Leeks and Onions they take the 

 place of flower-buds. Falling ofr', they take root and grow mto new plants. 



110. The Illteraal 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 wood, and much cellular tissue ; those of shrubs and trees 

 abound in the woody part. 



111. There are two great classes of stems, which difi^er in the way the woody 

 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, 

 Sunflower, or the like, among herbs, and for woody stems any common stick 

 of w^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 ways in which the two kinds grow in thickness 

 when they live more than one year. But the ditfei-ence 

 between the two is almost as apparent the first year, and 

 in the stems of herbs, which last only one year. 



113. The EndogCElOUS Stem. Endogenous means "inside- 

 growing." Fig. 77 shows an Endogenous stem in a Corn- 

 stalk, both in a cross-section, at the top, and also split 

 down lengthwise. The peculiarity is that the wood is all 

 in separate threads or bundles of fibres running lengthwise, 

 and scattered amon<]f the cellular tissue throu":hout the 

 whole thickness of the stem. On the cross-section their 

 cut ends appear as so many dots ; in the slice lengthwise 

 they show themselves to be threuds or fibres of w^ood. 

 Fig. 78 is a similar view of a Palm-stem (namely, of our 

 Carolina Palmetto, of which whole trees are represented 

 in Fig. 79). It shows the endogenous plan in a stem 

 several years old. Here the bundles of wood are merely 

 increased very much in number, new threads having been Endo-enoui stems. 

 formed throughout intermixed with the old, and any in- 

 crease ia diameter that has taken place is from a general distention or enlargement 



