; ) THE STEM 139 
the parenchyma to stretch them. These two strains, working 
against each other, produce rigidity in somewhat the same way 
as in a turgid cell, in which the con- 
tents, which have a tendency to swell, 
are compressed by the stretched cell 
wall, or as in a rubber tube when air 
or water is compressed within the 
tube. The compression of inner by 
outer tissues can very easily be dem- 
onstrated with stems or petioles of 
many herbaceous plants. Fig. 136 
shows a piece of a large petiole which 
was cut off evenly at both ends, after 
which the outer and inner parts were 
outside of the vascular bun- Ge eeew ab theupper end by eke 
RM rine of collanchyma lindrical cut. The central portion, 
(dotted area) jist within the which was under compression, being 
epidermis freed from the 
outer part, elon- 
gated and so projected beyond the latter, 
which contracted slightly. 
Mechanical tissues in monocotyledonous 
stems. In many monocotyledonous plants 
the arrangement of the strengthening ma- 
terial is very similar in principle to the 
reénforcing of concrete in a concrete struc- 
ture. The concrete withstands compression, 
while the iron rods withstand the tension 
due to movement etc. In monocotyledon- ¥16-186. Demonstra- 
ous plants the parenchyma withstands the ore eS irae ane 
: : . in a petiole of ele 
compression, while the sclerenchyma strands, Pranaanr etm 
which are connected with the vascular giants 
bundles, withstand the tension (Fig. 125). ; 
A very excellent example of this is abaca, or Manila hemp. This 
plant, whose appearance and structure are almost identical with 
those of the banana (Fig. 66), has a massive trunklike portion 

Fie. 135. Diagram of a dicoty- 
ledonous stem with a scleren- 
chyma ring (hatched area) 


