THE ANGIOSPERMAE : STEMS 885 



Even in plants with a meristematic ring which gives rise to a woody stem, 

 differentiation normally begins at a number of separate points on the ring, 

 so that separate primary bundles are for a short time distinguishable. The 

 few exceptions, such as Vim a (Fig. 871), in which a continuous ring of 

 xylem is present from the beginning, may really be analyzed into a ring 

 consisting of a very large number of uniseriate bundles, placed very close 

 together and showing a radial arrangement of the elements from the start. 



The residual meristem between the original bundles may organize directly 







'U^ ^^} *^ - m' ^ ■ '•• "ft^J 



Fig. 871. — Vinca major. Transverse sec- 

 tion of stem showing continuous zone 

 of xylem. 



into interfascicular cambium, continuous with that in the bundles, or it 

 may differentiate into parenchyma. In the first case secondary xylem and 

 phloem are differentiated in the spaces between the original bundles and 

 secondary growth goes forward both in the bundles and between them, 

 forming continuous zones of xylem and of phloem, as in timber trees. In 

 the second case a layer of the interfascicular parenchyma may later differentiate 

 into cambium, but such secondary cambium forms only or mainly parenchyma, 

 and the original bundles remain permanently distinct, i.e., secondary vascular 

 tissue is confined to the bundles. This is not uncommon in woody climbers 

 like the Vine. On the other hand, the spaces between the bundles may remain 

 permanently parenchvmatous, with no interfascicular cambium at all, and 

 secondary growth of the bundles themselves is very limited. This is the 

 extreme herbaceous type of stem (see p. 856). 



The parenchymatous intervals between the original bundles are the 

 primary medullary rays. They are usually broad at first, and in stems of 

 the Vine type they grow radially by additions from the interfascicular 



