DIFFERENTIATION OF TISSUES 287 



box or a "sieve." These perforated areas are the sieve- 

 plates, and through them the vessels communicate with 

 one another and with the adjacent tissue. 



The tracheary and sieve vessels occur in separate 

 strands, the tracheary strand being called xylem (" wood "), 

 the sieve strand phloem ("bark "). A xylem and a phloem 

 strand are usually organized together to form a vascular 

 bundle, and it is these fiber-like bundles which are found 

 traversing the stems of all vascular plants and appearing 

 conspicuously as the veins of leaves. Among the Dicotyls 

 and Conifers the vascular bundles appear in the stele in 

 such a way as to outline a hollow cylinder (Fig. 216), the 

 xylem of each bundle being toward the center, the phloem 

 toward the circumference of the stem. The undifferenti- 

 ated parenchyma of the sfcele which the vascular cylinder 

 incloses is called the pith. In older parts of the stem the 

 pith is often abandoned by the activities of the plant, and 

 either remains as a dead spongy tissue, or disappears en- 

 tirely, leaving a hollow stem. Between the bundles form- 

 ing the vascular cylinder there is also undifferentiated 

 parenchyma, and as it seems to extend from the pith out 

 between the bundles like "rays from the sun," the rays 

 are called pith rays. 



Such vascular bundles as described above, in which the 

 xylem and phloem strands are " side-by-side " upon the same 

 radius, are called collateral (Fig. 270). One of the pecul- 

 iarities of the collateral bundles of Dicotyls and Conifers, 

 however, is that when the two strands of each bundle are 

 organized some meristem is left between them. This means 

 that between the strands the work of forming new cells can 

 go on. Such bundles are said to be open ; and the open 

 collateral bundle is characteristic of the stems of the Dico- 

 tyls and Conifers. 



The meristem between the xylem and phloem of the 

 open bundle is called cambium (Figs. 268, 270). The cam- 

 bium also extends across the pith rays between the bundles, 



