138 



VEGETABLE LIFE AND WORK. [SECTION 16. 



needle-shaped (rhaphides), as in stalks of Calla-Lily, Rhubarb, or Four- 

 o'clock, they are usually packed in sheaf-like bundles. (Fig. 465, 466.) 



| 3. ANATOMY OF ROOTS AND STEMS. 



423. This is so nearly the same that an account of the internal structure 

 of stems may serve for the root also. 



424. At the beginning, either in the embryo or in an incipient shoot 

 from a bud, the whole stem is of tender cellular tissue or parenchyma. 

 But wood (consisting of wood-cells and ducts or vessels) begins to be 

 formed in the earliest growth ; and is from the first arranged in two ways, 

 making two general kinds of wood. The difference is obvious even in 

 herbs, but is more conspicuous in the enduring stems of shrubs and 

 trees. 



425. On one or the other of these two types the stems of all phanero- 

 gamous plants are constructed. In one, the wood is made up of separate 

 threads, scattered here and there throughout the whole diameter of the 

 stem. In the other, the wood is all collected to form a layer (in a slice 

 across the stem appearing as a ring) between a central cellular part which 

 has none in it, the Pith, and an outer cellular part, the Bark. 



An Asparagus-shoot and a Corn-stalk for herbs, and a rattan for a 

 woody kind, represent the first kind. To it 

 belong all plants with monocotyledonous em- 

 bryo (40). A Bean-stalk 

 and the stem of any com- 

 mon shrub or tree rep- 

 resent the second; and 



to it belong all plants with dicotyledonous or polycotyledonous embryo. 

 The first has been called, not very properly, Endogenous, which means in- 

 side-growing ; the second, properly enough, Exogenous, or outside-growing. 

 427. Endogenous Stems, those of Monocotyls (40), attain their 

 greatest size and most characteristic development in Palms and Dragon- 

 trees, therefore chiefly in warm climates, although the Palmetto and some 



FIG. 471. Diagram of structure of Palm or Yucca. 472. Structure of a Joid- 

 stalk, in transverse and longitudinal section. 473. Same of a small Palm-stem. 

 The dots on the^ cross section* represent cut ends of the woody bundles or threads. 



