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have been developed. They are especially fitted for giving 
strength to young, growing organs, since the thickened parts of 
the walls have considerable rigidity, while the thinner parts allow 
for an exchange of materials between cells and for the stretching 
and growth of the cells. The collenchyma cells of stems some- 
times contain chloroplastids and carry on photosynthesis. 
Sclerenchyma. Thick-walled dead cells known as sclerenchyma 
are found in the cortex of some stems. There are two varieties of 
these sclerenchyma cells: short or ir- 
regularly shaped cells, known as stone 
cells, and sclerenchyma fibers. Scle- 
renchyma fibers are long, dead, thick- 
walled cells and serve as strengthening 
material. Stone cells give stiffness to 
the cortex and, when they form a 
more or less continuous layer, may 
take the place of the epidermis after 
the part of the cortex which is exte- 
rior to them has disappeared. 
Stele of dicotyledonous plants. The 
ELIT. Wood Ahem ie oes of the stem inside of the cortex 
oak (Quercus bennettii). (x 40) 18 known as the stele (Figs. 113-115). 
This consists of three general regions. 
In the center is the pith, composed of thin-walled parenchyma 
cells. This pith is surrounded by the vascular-bundle region, which 
is composed of vascular bundles separated by radial rows of paren- 
chyma cells known as pith rays. Outside of this is the pericycle. 
Pith. In a dicotyledonous plant the center of the stem is 
composed of thin-walled parenchyma cells and is known as the 
pith (Figs. 118, 115). The cells of the pith are very similar to 
other parenchyma cells and have the same general functions. 
As they are not exposed to the light, they do not develop chloro- 
plastids and so do not perform photosynthesis. In perennial 
plants the pith is frequently a transitory structure. 
Vascular bundles. Around the pith are compact groups of cells 
which, as seen in cross section, are arranged in the general form 
120 A TEXTBOOK OF GENERAL BOTANY 

