244 - Multicellular Plants 



be replaced by the encroachment of the thick 

 cell walls, which impart mechanical strength 

 to the tissue. Sclerenchyma is present in many 

 stems and roots, and in some leaves. 



(b) Bast. This special type of scleren- 

 chyma is found in the phloem (p. 243) of 

 many stems (Fig. 13-8). Initially each bast 

 fiber is a living cell, which develops an elon- 

 gate spindlelike shape and very thick walls. 

 A mature bast fiber finally displays a small 

 central hollow, which is devoid of any proto- 

 plasm (Fig. 13-8). The pointed ends of the 

 spindle-shaped bast fibers tend to overlap 

 each other, imparting great flexibility and 

 strength to the tissue as a whole. 



(c) Wood likewise is a special type of scler- 

 enchyma, which is found in the xylem (p. 

 243) of many stems (Fig. 13-8). Wood fibers 

 resemble bast fibers in origin and form. How- 

 ever, in wood the cell walls are denser, and 

 this imparts a "harder" quality to the tissue. 



6. Vascular Tissues, (a) Sieve tubes are tubu- 

 lar units found in the phloem of stems (Fig. 

 13-8), roots, and leases. Fach sieve tube 

 represents a column of elongate cylindrical 

 cells, placed end to end. At maturity the end 

 walls of these cells become perforated, form- 

 ing the sieve plates, which give protoplasmic 

 continuity between the successively placed 

 cells in the column (Fig. 13-8). Moreover, the 

 cytoplasm of a mature sieve tube unit ap- 

 pears to be somewhat modified and no nu- 

 cleus is present. The protoplasm of the sieve 

 tubes displays active cyclosis, which acceler- 

 ates the distribution of substances lengthwise 

 through the plant. The sieve tubes are con- 

 cerned particularly with the transportation 

 of sugars and other organic products, down- 

 ward and sideways in the plant. 



(b) Ducts, or vessels, are tubular units local- 

 ized in the xylem regions of roots, stems, and 

 leaves. The initial differentiation of a duct 

 from embryonic parenchyma resembles that 

 of a sieve tube, but later the side wall of the 

 column of cells becomes much thicker, and 

 gradually the end walls are resorbed com- 

 pletely (Fig. 13-8). Finally all the protoplasm 

 disappears, leaving a free channel throughout 



the length of each vessel. The ducts form 

 continuous channels from the roots up 

 through the stem and into the leaves. Ducts 

 serve to transport substances — particularly 

 water and inorganic salts — upward through 

 the plant. Several kinds of ducts are distin- 

 guishable. In pitted ducts, the side walls are 

 pocked with numerous round depressions 

 (Fig. 13-8). These pits partially penetrate the 

 cellulose and permit substances to escape 

 from the ducts, supplying the surrounding 

 tissues at various levels of the plant. In spiral 

 (Fig. 13-8) and annular ducts, thickenings of 

 the side walls can be seen, which give a 

 "springlike" and a "ringlike" appearance to 

 each respective type. The xylem of some 

 vascular plants contains tracheids, which may 

 replace the ducts, either partially or com- 

 pletely. Generally speaking, tracheitis are like 

 ducts, except that their conducting channels 

 are interrupted periodically by diagonally 

 placed perforated cell walls. In addition to 

 the vascular function, ducts and tracheitis 

 augment the mechanical strength of the vari- 

 ous parts, especially in the stem. 



The Root and Its Functions. Aside from the 

 fact that some roots serve as repositories for 

 reserves of synthesized organic products, espe- 

 cially starch, the root system of a vascular 

 plant has two main functions: (1) it absorbs 

 water and salts for the plant as a whole; and 

 (2) it anchors the plant in an upright posi- 

 tion. Accordingly, the roots of a plant ramify 

 extensively throughout the soil. In many trees 

 the roots branch even more profusely than 

 the stem; and frequently the total root sur- 

 face is greater than that of the stem with all 

 its branches (Fig. 13-9). The root system ol 

 a single rye plant, grown under ideal condi- 

 tions, was found to measure more than 300 

 miles of branching length. These roots pro- 

 side a surface area of more than 2500 square 

 ieet, not counting the area of the root hairs. 

 However, the depth of roots seldom equals 

 the height of the stem, although frequently 

 the two systems have a corresponding lateral 

 spread. 



Not all parts of the root svstem are equally 



