me THE STEM 127 
are like other sclerenchyma cells (Fig. 119) in being long, thick- 
walled dead cells which serve as strengthening material. 
Variations in stem structure. The above description of the 
structure of stems applies to the great majority of dicotyledonous 
plants, but there are a few which show minor variations. If these 
few exceptional cases are 
left out of consideration, 
the general structure of 
dicotyledonous stems is 
remarkably uniform. The 
relative development of 
the various parts, how- 
ever, varies greatly in 
different species. In 
some cases the pith is 
wide, while in others it 
is narrow. It may be 
wide and transitory and 
its early disappearance 
result in a hollow stem. 
The vascular bundles 
vary considerably in 
number and size, while 
the pith rays and cortex 
vary in width. Thus, 
while there is a remark- 
able uniformity in the general plan of the dicotyledonous stem, 
there are at the same time considerable differences in detail. 
Bundles which have the phloém only on the outside of the 
xylem are called collateral bundles. The bundles of some plants 
have phloém on both the outside and the inside of the xylem 
(Fig. 120) and are known as bicollateral bundles. 
Resin ducts. Many plants contain intercellular passages filled 
with resinous substances and known as resin ducts. They are 
formed either by the separation of neighboring cells (Fig. 121) 
or by the disintegration of cells. Turpentine is obtained by cutting 
a 

Fic. 124. Network of latex tubes of a rubber 
tree (Manihot glaziovil) 

