. Bulletin - Massachusetts Agricultural Experiment Station. Agriculture -- Massachusetts. ROSE CANKER AND ITS CONTROL. 27 strength. The areas of bast fibers do not form a complete cylinder, but the inner cortex tissue runs down between them. Just under each bast area there is a region of tissue called phloem. It contains long tubes (sieve tubes) through which the elaborated plant food passes down through the stem from the leaves. Each sieve tube is accompanied by a line of small slender cells (companion cells), which appear in transection as though they were cut out of the corners of the sieve
. Bulletin - Massachusetts Agricultural Experiment Station. Agriculture -- Massachusetts. ROSE CANKER AND ITS CONTROL. 27 strength. The areas of bast fibers do not form a complete cylinder, but the inner cortex tissue runs down between them. Just under each bast area there is a region of tissue called phloem. It contains long tubes (sieve tubes) through which the elaborated plant food passes down through the stem from the leaves. Each sieve tube is accompanied by a line of small slender cells (companion cells), which appear in transection as though they were cut out of the corners of the sieve tubes. The remain- ing cells of the phloem are box-like cells called phloem parenchyma. The phloem is bounded below by the cylinder of thin flat cells, the cambium, which marks the line of cleavage between the bark and wood. The wood, or xylem, is composed mostly of four kinds of cells: (1) Box-like parenchyma cells which compose the broad medullary rays as well as the narrow rays one cell in width. (2) Long tubes of large diam- eter (tracheae) through which the water mainly passes from the roots to the parts above. The walls are strengthened bj^ spiral or annular thickenings. (3) Vertically elongated cells (tracheids) of smaller diameter and thicker w^alls, also water carriers. These make up the greater portion of the wood. (4) Wood fibers, somewhat smaller in diameter, with thick walls and long tapering points. They cannot be distinguished from the tracheids in transection. Although the walls of all the xjdem elements are heavj^ they are all marked with pits so that liquids have only a thin membrane through which they must pass to go from one cell to the next. The pith (not shown in the figure) is composed of cells of only one kind, large or small, somewhat isodiametric (parenchyma). The walls are very thin. Path of the Mycelium. â The germ tube, when it attacks the host, is very slender and easily passes between the guard cells down into the stomatal cavity. It could then
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