112 THE STEM. 



ope. That of the Colchicum produces a new bud near the base of 

 the old, upon which it feeds, and is in turn destroyed by its own 

 progeny the next year ; so that we observe (Fig. 149), a, the shriv- 

 elled corm of the year preceding ; Z>, that of the present season (a 

 vertical section) ; and c, the nascent bud for the ensuing season. 



180. Many of the forms which the stem assumes when above 

 ground differ as much from the ordinary appearance as do any of 

 these subterranean kinds ; as, for example, the globular Melon-Cac- 

 tus, the columnar Cereus, and the jointed Opuntia or Prickly Pear. 



SECT. IV. THE INTERNAL STRUCTURE OF THE STEM IN GENERAL. 



181. Having considered the various external forms and appear- 

 ances which the stem exhibits, and its mode of increase in length, 

 our attention may now be directed to its internal structure, and 

 mode of increase in diameter. 



182. The stem embraces in its composition the various forms of 

 elementary tissue that have already been described (Chap. I., Sect. 

 II., III.) ; namely, ordinary cells, woody fibre, and vessels. At 

 first, indeed, it consists entirely of parenchyma (51), which pos- 

 sesses much less strength and tenacity than woody tissue, and is 

 therefore inadequate to the purposes for which the stem, in all the 

 higher plants, is destined. The stem of a Moss or a Liverwort is, 

 in fact, composed of ordinary cellular tissue alone ; and is there- 

 fore weak and brittle, well enough adapted to the humble size of 

 that tribe of plants, but incapable of attaining any considerable 

 height. Accordingly, as soon as the stems of all the Phsenogamous 

 plants begin to grow, and in proportion as ihe leaves are developed, 

 woody mingled with vascular tissue is introduced, woven into the 

 original cellular fabric, to afford the requisite toughness and 

 strength, and to facilitate the rise of the ascending sap. If it 

 accumulates only to moderate extent in proportion to the paren- 

 chyma, the stem remains herbaceous (159) ; if it predominates and 

 continues to accumulate from year to year, the proper woody trunk 

 of a shrub or tree is formed. That the woody and vascular tissues 

 arise from cells, which from an early period take a peculiar devel- 

 opment, has already been shown (52-61). 



183. The cellular part of the stem grows with equal readiness, 

 in whatever direction the forces of vegetation act. It grows verti- 

 cally, to increase the stem in length, and horizontally, to increase 



