. Plant life, considered with special references to form and function. Plant physiology. THE STEM. "S concentric with the surface of the stem (figs. 120, 124, 133^-!). As its cells divide, sometimes their inner, sometimes their outer segments differentiate into the tissues which they then. sn^" Fig. 133.—Diagrams of transverse sections of stems illustrating modes of secondary thickening. In all f, cortex; cii. endoderniis ; /, pericycle ; //;', primary phloem ; J'lt' , secondary phloem; ch^ stelar ; j^', primary xvlem ; ;, secondary xylem ; ?-', primary pith rays ; r&
. Plant life, considered with special references to form and function. Plant physiology. THE STEM. "S concentric with the surface of the stem (figs. 120, 124, 133^-!). As its cells divide, sometimes their inner, sometimes their outer segments differentiate into the tissues which they then. sn^" Fig. 133.—Diagrams of transverse sections of stems illustrating modes of secondary thickening. In all f, cortex; cii. endoderniis ; /, pericycle ; //;', primary phloem ; J'lt' , secondary phloem; ch^ stelar ; j^', primary xvlem ; ;, secondary xylem ; ?-', primary pith rays ; r", secondary pith rays—Alter Van Tieghem. adjoin. Inside the generating layer between the bundles there arises, therefore, secondary xylem which becomes wood ; outside it, secondary phloem, or bast. Each bundle is thus increased in its radial dimension (fig. 124J. 142. Pith rays.—The generating layer in the pith rays arises from the pericycle or from some part more deep- seated, but in any case it connects directly with the generat- ing layer between the adjacent bundles (fig. 124). In this portion of the generating layer two distinct modes of develop- ment are to be observed ; either tlie tissues produced by the division of the cells differentiate into pith tissue {B, fig. 133), or they form secondary wood and liast coresponding to that produced between the adjacent bimdles. In tlie latter case, therefore, a complete zone or ring of secondary wood and bast is formed, so that the pith occupies the center. Upon the ring of secondary wood thus produced the primary wood bundle projects into the pith, and upon the ring of secon- dary bast the primary bast bundle projects into the cortex (C, fig. 133).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Barnes, Charles Reid, 1858-1910. New York, H. Holt &a
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