INTERNAL STRUCTURE OF STEMS. 



41 



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

 ground, as in the Bulb-bearing Lilv. Or in some Leeks and Onions thej take the 

 place of flower-buds. Falling off, they take root and grow into new plants. 



110. TllC Internal Structure of Stems. Plants are composed of two kinds of ma- 

 terial, namely. Cellular Tissue and ]Vood. The former makes the softer, fleshj-, luid 

 pithy parts ; the latter forms the harder, fibrous, or woody parts. The stems of 

 herbs contain little wood, and nmch 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 Exor/enouSj 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 wavs in which the tv/o kinds ":rowin thickness 

 when they live more than one year. But the difference 

 between the tvv'o is almost as apparent the first year, and 

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



113. The Endogenous Stem. Endorjenous 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 v.-ood is all 

 in separate threads or bundles of fibres running lengtlnvise, 

 and scattered amonsr the cellular tissue throufrliout 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 witli tlie old, and any in- 

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



Endo'enouj Stems. 



