How Substances are Transported and Removed 221 



while a typical stem is shown generahzed in our later figure 139 B. 

 Among the many cellular elements in the symmetrical, almost 

 geometrical structure thus displayed, it is easy to identify the 

 bundles of ducts from their relatively large size and their obvious 

 resemblance to the cut ends of 

 round tubes. Associated with 

 the ducts, and a little way re- 

 moved towards the outside of 

 the stem, lie clusters of smaller, 

 thinner-walled, and more angu- 

 lar cells, which are also the cut 

 ends of long tubes, the food- 

 carrying sieve-tubes] while be- 

 tween sieve-tubes and ducts lie 

 two or three layers of small 

 squarish cells presenting an 



aspect which later the reader ^^^ 73.-Cross section of a young stem of 



will learn to associate with t^e Castor Bean, magnified about twenty 



times. (Copied, reduced, from a drawing 

 growth, for they are the cam- by H. O. Hanson, in Curtis' Xature and 

 7 . ,. 1 • 1 p 1 , Development of Playits.) 



bium cells which form new ducts 



and sieve-tubes as long as the plant lives. Ducts, sieve-tubes and 

 cambium, to which often are added strengthening fibers, grow 

 all or a part of them together in bundles, forming fibro-vascular 

 bundles which are identical with the veins,- — both the kind that 

 can be seen in young translucent stems, and also those familiar 

 in leaves. The bundles begin, as our generalized picture of the 

 conducting system illustrates (figure 54), near the ends of the 

 roots, where they consist of a few ducts and sieve-tubes only; 

 farther back they acquire cambium and fibers and enlarge greatly 

 in size; in the stem they branch at the nodes and run out to the 

 leaves, when they fringe away gradually to the veinlets, each of 

 which ends as a single duct and sieve-tube in the midst of one of 

 the ultimate areas of green tissue. 



The fibro-vascular bundles have not only this definite com- 



