ELEMENTS OF ORGANIC STRUCTURE. 227 
nucleus, but are filled with a sap or juice containing citric 
acid and sugar. 
In the pith of the rush, star-shaped cells are found. In 
common mould the cells are long and 
thread-like. In the so-called frog-spittle 
they are cylindrical and attached end to 
end. In the bark of many trees, in the 
stems and leaves of grasses, they are 
square or rectangular. 
Cotton-fiber, flax and hemp consist of 
long and slender cells, fig. 31. Wood is 
mostly made up of elongated cells, tapered 
at the ends and adhering together by 
their sides. Fig. 49, ¢. h., p. 271. 
Each cotton-fiber is a single cell which forms an 
external appendage to the seed-vessel of the cot- 
ton plant. When it has lost its free water of 
vegetation and become air-dry, its sides collapse 
and it resembles a twisted strap. <A, in fig. 31, 
exhibits a portion of a cotton-fiber highly magnified. 
The flax-fiber, from the inner bark of the flax- 
stem, 0, fig. 31, is a tube of thicker walls and 
smaller bore than the cotton-fiber, and hence is more durable than cot- 
ton. It is very flexible, and even when crushed or bent short, retains 
much of its original tenacity. Hemp-fiber closely resembles flax-fiber in 
appearance. 
Thickening of the Cell-Wembrane.—The growth of the 
cell, which, when young, always has 
avery delicate outer membrane, often 
results in the thickening of its walls 
by the interior deposition of cellu- 
loseand lignin. This thickening may 
take place regularly and uniform- 
ly, or interruptedly. The flax-fiber, 
6, fig. 31, is an example of nearly 
uniform thickening. The irregular 
deposition of cellulose is shown in 
fig. $2, which exhibits a section from 
the seeds (cotyledons) of the com- 
mon nasturtium, (Zropeolum majus). The original membrane is coated 
interiorly with several distinct and successively-formed linings, which 
are not continuous, but are irregularly developed. Seen in section, the 
