112 



THE STEM. 



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 wliich 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 therefore 

 weak and brittle, well enough adapted to plants of humble size, 

 but not for those which attain any considerable height. Accord- 

 ingly, as soon as the stems of Phajnogamous plants begin to grow, 

 and in proportion as the leaves are develojjed, woody mingled with 

 vascular tissue is introduced, to afford the requisite toughness and 

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

 wood accumulates only to moderate extent in proportion to the 

 parenchyma, the stem remains herbaceous (174) ; if it predominates 

 and continues to accumulate from year to year, the projier woody 

 trunk of a shrub or tree is formed. 



197. 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 

 its diameter. Into this the elongated cells that form the woody 

 tissue and ducts are introduced vertically ; they run lengthwise 

 through the stem and branches. Hence, the latter has been called 

 the longitudinal, vertical, or perpendicular system (56, 64) ; and 

 the cellular part, the horizontal system of the stem. Or the stem 

 may be compared to a web of cloth ; the cellular system forming 

 the woof, and the woody, the warp. 



198. The diversities in the internal structure of the stem are 

 principally owing to the different modes in which the woody or 

 vertical system is imbedded in the cellular. These diversities are 

 reducible to two general plans ; upon one or the other of which the 

 stems of all Flowering plants are constructed. Not only is the 

 difference in structure quite striking, especially in all stems more 

 than a year old, but it is manifested in the whole vegetation of the 

 two kinds of plants, and indicates the division of Phsenogamous 

 plants into two great classes, recognizable by every eye ; which, 

 in their fully developed forms, may be represented, one by the Oak 

 and other trees of our climate, the other by the Palm (Fig. 184). 



199. The difference between the two, as to the structure of their 

 stems, is briefly and simply this. In the first, the woody system is 

 deposited in annual concentric layers between a central pith and an 

 exterior baric; so that a cross-section presents a series of rings or 



