. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. THE PRIMARY PERMANENT TISSUES 35 them is intended to reduce permeability between primary cortex and stele; and this conception is strengthened by the occurrence, in such an endodermis, of thin-walled cells just in front of the xylem portion of roots where water absorbed from the soil has need of access to the water tubes (Fig. i6). In old portions of roots it often happens that the outer parts of the primary cortex slough away, leaving the endodermis to protect the
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. THE PRIMARY PERMANENT TISSUES 35 them is intended to reduce permeability between primary cortex and stele; and this conception is strengthened by the occurrence, in such an endodermis, of thin-walled cells just in front of the xylem portion of roots where water absorbed from the soil has need of access to the water tubes (Fig. i6). In old portions of roots it often happens that the outer parts of the primary cortex slough away, leaving the endodermis to protect the stele. The Pericycle.—The pericycle lies between the starch sheath or endodermis and the outer rim of the phloem part of the vas- cular bundles (Fig. 14). In stems we commonly find it composed of two kinds of tissues, thin-walled parenchyma and bast fibers, the origin of which from the ground meristem is as stated for the corresponding tissues in the primary cortex. The bast fibers. Fig. r 7.—Diagram to show different plans in the distribution of bast fibers. A, bast a -continuous cylinder in the pericycle; B, isolated strands of bast in the cortex and in the pericycle in front of each vascular bundle; C, a combination of A and B. (After Green.) may form a continuous zone all around the stem, or they may occur as isolated groups, either associated with, and seemingly a part of the phloem of the bundles, or dissociated from the phloem {Fig. 17). In the stems of most dicotyledonous plants the bast fibers are restricted to the pericycle. They serve, of course, for giving strength; and unless, or until, the cambium later adds a substantial amount of wood fibers or fiber tracheids they remain the chief reliance in this respect. The thin-walled parenchyma cells of the pericycle, like those of the primary cortex, often contain chloroplasts, and they serve for the slow conduction and storage of reserve foods, particularly of the non-nitrogenous class. In roots the pericycle occurs
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