INTERNAL STRUCTURE OF STEMS. 



41 



109. BulbletS are little bulbs, or flesliy buds, formed in the axils of leaves above 

 ground, as in the Bulb-bearing Lilj. Or in some Leeks and Onions they take the 

 place of flower-buds. Falhng 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 wood, and much cellular tissue ; those of shrubs and trees 

 abound in the woody part. 



111. There are two great classes of stems, which differ in the way the woody 

 part is arranged in the cellular tissue. They are named the JExogenous, 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 wood. 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 difference 

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

 in the stems of herbs, ^vhich last only one year. 



113. The Endogenous 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 among the cellular tissue throughout 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 threads or fibres of wood. 

 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 

 formed throughout intermixed with the old, and any in- 

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



Endogenons Steins. 



