. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. l62 TRANSPORT OF FOODS Relation of Phloem Elements to Other Tissues.—The food in its downward or upward course through the phloem may be drawn upon at any point by hving tissues for growth, repair, etc., or it may be set aside for storage in medullary rays, xylem and phloem parenchyma. Evidently to facilitate this movement of food-stuffs there are often pores between sieve tubes and companion cells, and thin places or pits where the medullary rays abut on com- pani
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. l62 TRANSPORT OF FOODS Relation of Phloem Elements to Other Tissues.—The food in its downward or upward course through the phloem may be drawn upon at any point by hving tissues for growth, repair, etc., or it may be set aside for storage in medullary rays, xylem and phloem parenchyma. Evidently to facilitate this movement of food-stuffs there are often pores between sieve tubes and companion cells, and thin places or pits where the medullary rays abut on com- panion cells or phloem par- enchyma. Pits also occur in the tangential walls of the medullary rays to help along the radial movements and storage of foods in the medul- lary rays; and pits in the walls separating the rays from wood parenchyma cells assist in the transmission of foods to, and storage in, the latter (Fig. 88). The medullary rays have for their primary function the radial transmission and storage of food. Their intimate relation with the cells of the phloem at their outer end and with the xylem parenchyma along their inner course, and the fact that we usually find them gorged with food, points to this conclusion. The short vertical extent of the rays, and their isolation from each other, renders them unsuited for the vertical or longitudinal transmission of foods. If they were of value in this respect girdling would not prevent the downward flow of foods. The extreme frequency of the rays is one factor of great im- portance to their efficiency in radial conduction and storage. In tangential section, that is, as one would see it when facing. Fig. 88.—Showing pitted connections between medullary rays and xylem paren- chyma, and between contiguous xylem parenchyma cells. m, medullary rays; n, xylem parenchyma. Camera-lucida drawing of tangential section of wood of yellow Please note that these images are extracted from scanned page images that may have been
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectplantanatomy, bookyea
