. Botany for high schools and colleges. Botany. lar and interfascicular cambium layers are composed of elongated cells, which multiply by fission in a tangential di- rection, and thus give rise to radiating rows of cells (Figs. 324 and 325). In a tangential section the cambium cells present an elongated outline, and their extremities are usually more or less oblique (Fig. 326). From these cells there develop various tissues. Thus^ on the one side, the phloem parenchyma, sieve and fibrous tissues may be pro- duced by more or less great modifications (Fig. 327). On ^^ /• , the other side (the xy
. Botany for high schools and colleges. Botany. lar and interfascicular cambium layers are composed of elongated cells, which multiply by fission in a tangential di- rection, and thus give rise to radiating rows of cells (Figs. 324 and 325). In a tangential section the cambium cells present an elongated outline, and their extremities are usually more or less oblique (Fig. 326). From these cells there develop various tissues. Thus^ on the one side, the phloem parenchyma, sieve and fibrous tissues may be pro- duced by more or less great modifications (Fig. 327). On ^^ /• , the other side (the xylem side) new ves- sels, fibres, and parenchyma are also devel- oped (Fig. 328). The development of IP=—^: these tissues begins in the inner and outer (____3^ layers of the cambium, and advances to- ward the central layers. It never hajJ- pens, however, that all the cambium lay- v,;:^^—XV ers pass over into permanent tissues, there I ^r always remaining one or a few meristem ' ^ layers. 546.—A study of Figs. 326-328 will show the probable mode of development of the permanent tissues from the meristem tissue of the cambium. It is evident from a comparison of Figs. 326 and 327 that the phloem parenchyma is produced by the formation of several transverse parti- tions in each cambium cell, and it is prob- able that in many cases there is a direct^ conversion of cambium cells into sieve tubes. That the cambium cells may be converted directly into tracheides is evident from Fig. 326, and also Fig. 75 (p. 84). In Fig. 328 it is plain that the fibrous tissue {If) and tracheides {t) have the same origin, and the indications are that even the large pitted vessels {gg) are formed from cambium cells by the great increase in the diameter of the latter, the thickening of their vertical walls, and the partial or complete absorption of their trans- verse wfills. The origin of the xylem parenchyma from cam-. Fig. 325.—The row of cells marked a? —a? in Fig. 324; r. phloem : h, xylem
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