188 A TEXTBOOK OF GENERAL BOTANY 
strands on opposite sides of the stem represent the flanges of an 
I-beam. This sclerenchyma, which is near a bundle, not only 
serves as one of the flanges of an I-beam but also is in a position 
to protect the delicate elements of the phloém. When the edges 
of the flanges of such a compound girder as that shown in 
Fig. 132 are connected, there is no necessity for the webs; and 
if they are absent, a hollow cylinder results. This type of con- 
struction is very frequently found in plants and is particularly 
evident in hollow stems, such as 
those of most grasses. In many 
plants the sclerenchyma is arranged 
in the form of a hollow cylinder 
(Figs. 113, 185). Such hollow cyl- 
inders are, as a rule, near the outer 
surface of the stem. The strength- 
ening material of the growing part 
of a stem is usually collenchyma, 
and this is generally arranged in the 
Fie. 134. Diagram of a dicoty- form of a hollow cylinder just within 
ledonous stem, showing scle- the epidermis (Figs. 134, 135). 
renchyma (hatched areas) just The foregoing discussion of the 
pS ee ae he ae arrangement of the strengthening - 
lenchyma (dotted area) forming A a 
a hollow cylinder within the terial in the stem applies partic- 
epidermis » ularly to herbaceous and young 
stems. In the case of trees the de- 
velopment of a massive column of wood takes the place of the » 
strengthening material that was used in the young stem. 
Tissue tensions. The location of thick-walled mechanical cells 
near the periphery of an organ with thin-walled parenchyma cells 
in the center gives rigidity in another manner besides that just 
considered. Thick-walled cells, particularly dead sclerenchyma 
cells, offer considerable resistance to any tendency to stretch 
them. On the other hand, thin-walled parenchyma cells tend to 
stretch, owing to their turgidity. The stretching of the paren- 
chyma is, however, checked by the strong, thick-walled cells, 
which are themselves under tension, owing to the tendency of 

