220 



Practical Plant Biology. 



about a dozen or less of these narrow cells in its cross section. 

 The constituents of the primary bast are* of two kinds wide 

 prismatic tubes and narrow cells. The former have transverse end- 

 walls. They are lined with a protoplasmic layer of great tenuity 

 and contain a watery protein-sol and dissolved sugars. In the 

 protoplasmic film may be detected small fragments of nuclear 

 substance, and also minute starch grains. The end-walls of these 

 tubes are perforated and act as sieve-plates. Through these per- 

 forations the contents of adjacent sieve-tubes are continuous. 

 Often sieve-plates are formed on the longitudinal walls where two 

 tubes are in contact with one another. The other constituents of 



FIG. 76. Ranunculus repens, conducting tract, longitudinal section, x 250. 

 6, bast; c, cortex; m, pith ; px, protoxylem ; s, sheath ; x, wood. 



the primary bast are the companion cells. They lie between the 

 sieve-tubes, and are of the same length as they are : but in cross 

 section they are much smaller. Their walls are equally thin, but 

 their cavities are practically filled with protoplasm, leaving no 

 visible vacuole. A large elongated nucleus completes their contrast 

 with the sieve-tubes. A few flat tubular cells of equal length with 

 the sieve-tubes lie on the inner surface of the bast. They are 

 procambial cells, which have failed to differentiate and have lost 

 the power of dividing. 



The wood or xylem, like the bast, is composed of two parallel 

 filaments. That nearest the axis of the stem is the protoxylem. 



