

THE STEM 138 - 
there are frequently two elements of the protoxylem ; the first 
is an annular vessel and the second is a spiral vessel. On both 
the right and the left side of the bundle is found a large 
vessel which is pitted or narrowly reticulate. Between the 
larger vessels are smaller, tracheary elements. The phloém is 
next to the peripheral part of the bundle sheath and contains 
conspicuous sieve tubes and companion cells. 
The number of vessels varies in monocoty- 
ledonous bundles. In many cases there is 
only one large vessel, instead of two as in 
corn and sugar cane. 
COURSE OF BUNDLES IN STEMS 
A bundle extends up the stem and then 
passes out into a leaf. The bundles from 
leaves in the upper part of the stem enter 
the stem, pass down, and at a lower node 
join with other bundles. In this way the 
vascular elements in the stem become con- 
nected with new leaves. At the nodes bun- 
dles branch, while branches of different 
bundles coalesce (Fig. 129). 




Fie. 129. Course: of 
bundles in stem of 
In coniferous stems the general arrange- Clematis verticellaris 
ment of the various tissues is very similar Modified after Nagali 
+o that in dicotyledonous stems. The same 
type of secondary thickening occurs in both kinds of stems. 
The most conspicuous differences are that the xylem of conifers 
contains only tracheids and wood parenchyma, and that com- 
panion cells are not found in the phloém. 
Since the xylem of conifers does not contain fibers or vessels 
(Fig. 130), the tracheids serve both as water-conducting and as 
strengthening elements. Tracheids are not so well fitted for 
conducting water as are vessels. The small size and xerophytic 
ANATOMY OF CONIFEROUS STEMS 
