34 A TEXTBOOK OF GENERAL BOTANY 
Collenchyma. There are two ways in which cells give strength 
to plant organs. In some cases the cell walls are very greatly 
thickened, and the thickened cell walls give strength. This is 
true of the wood of woody stems. Other cells become stretched 
by the water which is in them, and therefore are rigid. Such 
cells may serve to strengthen organs. Cells which are stretched 
by the water in them are said to be turgid. The rigidity de- 
rived from turgidity has some points of similarity to that of a 
hose full of water, or that of an 
automobile tire filled with air 
under pressure. 7 
In the center of the upper por- 
tion of the midrib, just below the 
epidermis, there is usually a group 
of cells which give strength, both 
by having thickened walls and by 
being turgid. A group of the same 
kind of cells usually occurs also 
just above the lower epidermis. 
These cells are known as collen- 
chyma. In Fig. 24 the collenchyma 
is Shown as a crescent-shaped layer 
around the lower part of the midrib and just within the epider- 
mis; also as a small group of cells extending into the projection 
from the upper surface of the midrib. In the section shown in 
Fig. 26 the collenchyma is in the same position as in F ig. 24, 
but is less evident because the walls are not greatly thickened. 
Collenchyma is composed of living cells with walls which are 
thickened at the angles where three or more cells come in con- 
tact with one another. This is clearly shown in Fig. 25. The 
thick places in the walls increase the strength of the cells, while 
the thin places allow for a more rapid transfer of materials from 
cell to cell than would take place if the cell walls were thickened 
throughout. These cells are more or less turgid, and so give 
strength to the leaf in this way also. The turgidity likewise is 
illustrated in Fig. 25 above. 

Fig. 25. Cross section through the 
epidermis and collenchyma of a 
Coleus stem. (x 245) 
