272 HOW CROPS GROW. 
by an imperforate membrane. The wood and bast-cells, ¢, 
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, 
d; spiral ducts, e ; dotted ducts, f. These are continuous 
tubes produced by the resorption of the transverse mem- 
branes that once divided them into such cells as a, a, aud 
they are thickened internally by ring-like, spiral, or punce- 
tate depositions of cellulose, (see fig. 32, p. 227.) Wood- 
cells that consist exclusively of cellulose are pliant and 
elastic. It is the deposition of lignin in their walls which 
renders them stiff and brittle. 
At g. the cambium tissue is observed to consist of deli- 
cate cylindrical cells. Among these, partial resorption of 
the separating membrane often occurs, so that they com- 
municate directly with each other through sieve-like parti- 
tions, and become continuous channels or ducts, (sieve-cells, 
p. 280.) 
The cambium is the seat of growth by cell-formation. 
Accordingly, when a vascular bundle has attained maturi- 
ty, it no longer possesses a cambium; the latter has grown 
away from it, has reproduced itself in originating a new 
vascular bundle, which, in case of the endogens, branches 
off from the present bundle, and with exogens, runs paral- 
lel with, and exterior to the latter. 
To complete our view of the vascular bundle, fig. 50 
represents a vertical section made at right angles to the 
last, cutting two large ducts, 0,6, a, a, is cell-tissue; ¢ 
c, are bast or wood-cells less thickened by interior deposi 
tion than those of fig. 49; d, is a ring and spiral duct; 6, 
b, are large dotted ducts, which exhibit at g, g, the places 
where they were once crossed by the double membrane 
ecmposing the ends of two adhering cells, by whose ab- 
