394 HOW CKOPS GKOW. 



distinguished only by their position. In other plants, 

 they are often unlike as regards length, thickness, and 

 pliability, though still, for the most part, similar in 

 form. Among the wood-cells we observe a number of 

 ducts,, d, e, f, and between these and the bast-cells is a 

 delicate and transparent tissue, g, which is the cambium, 

 in which all the growth of the bundle goes on until it 

 is complete. On either hand is seen a remarkably large 

 duct, b, b, while the residue of the bundle is composed 

 of long and rather thick-walled wood-cells. 



Fig. 49 represents a section made Tertically through 

 the bundle from c to h. In this the letters refer to the 

 same parts as in the former cut : a, a is the cell-tissue, 

 enveloping the vascular bundle; the cells are observed 

 to be much longer than wide, but are separated from 

 each other at the ends as well as sides by an imperforate 

 membrane. The wood and bast-cells, c, h, are seen to 

 be long, narrow, thick-walled cells running obliquely to 

 a point at either end. The wood-cells of oak, hickory, 

 and the toughest woods, as well as the bast-cells of flax 

 and hemp, are quite similar in form and appearance. 

 The proper ducts of the stem are next in the order of 

 our section. Of these there are several varieties, as ring- 

 ducts, dj spiral ducts, e; dotted ducts,/. These are 

 continuous tubes produced by the absorption of the 

 transverse membranes tliat once divided them into such 

 cells as a, a, and they are thickened internally by ring- 

 like, spiral, or punctate depositions of cellulose (see Fig. 

 32, p. 248). Wood or bast-cells that consist mainly of 

 cellulose are pliant and elastic. It is the deposition of 

 other matters (so-called lignin) in their walls which ren- 

 ders them stiff and brittle. 



At g, the cambial tissue is observed to consist of del- 

 icate cylindrical cells. Among these, partial absorption 

 of the separating membrane often occurs, so that they 

 communicate directly with each other through sieve-like 



