40 CILIATED AND BRANCHED CELLS. 
one another when combined together to formatissue. Another 
distinction commonly observed between parenchymatous and 
prosenchymatous cells arises from the condition of their cell- 
walls ; thus, those of parenchymatous cells are usually thin (figs. 
66 and 69), while those of prosenchymatous cells are more or 
less thickened (figs. 96 and 97). These latter cells are com- 
monly termed fibres. The above distinctions between paren- 
chymatous and prosenchymatous cells are evident enough in the 
extreme forms of the two divisions, but various transitional 
states occur which render it impossible to draw, in many cases, 
a distinct line of demarcation between them. 
When cells are so placed as to be uncombined with other 
cells, or with the vessels of the plant, or but partially so, they 
Bie. wes 
Fig. 72. A portion of the frond of Witophyllum laceratum. a, a. Cell walls. 
b, b. Contents of the cells. After H. B. Brady. 
are more or less unrestrained in their development ; but even in 
such circumstances, as in their combined state, their typical 
form is to be more or less rounded. This form is, however, 
rarely maintained as they grow older, although instances of such 
occur in many of the lower Algz, as Protococcus (jig. 1) ; in 
pollen cells (fig. 73) ; and in spores ; but more frequently, in 
such cases, the cells assume a more or less elongated form and 
become oblong (fig. 77), cylindrical (fig. 74), &c. Insuch cells, 
again, we frequently find that certain points of the cell-wall 
acquire a special development (see page 23), and become elevated 
from its general surface as little papille (fig. 73), warty projec- 
tions (fig. 74), or cilia (figs. 75, 76, and 77); or are prolonged 
into tubular processes, or branched in various ways (jig. 78). 
The hairs which are produced on the surface of plants afford good 
