272 HOW CROPS GROW. 



by an imperforate membrane. The wood and baat-cells, c, 

 hj 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 stein 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, and 

 they are thickened internally by ring-like, spiral, or punc 

 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, 

 [.. 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 latte r. 



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, b, b ; a, a, is cell-tissue; c 

 c, are bast or wood-cells less thickened by interior deposi 

 t:on than those of fig. 49; d, is a ring and spiral duct; 6, 

 &amp;gt;, are large dotted ducts, which exhibit at g, g, the places 

 where they were once crossed by the double membrane 

 composing the ends of two adhering cells, by whose al&amp;gt; 



