< - 
ee 12 ‘Structural ond Ph ‘issiolop teal Bona” «ie i ibs 
The cs ane size of the naked cell (without on wall) 
_ are very various, in consequence of the constantly changing 3 
motion of the protoplasm. In those which are enclosed in 
a firm wall or membrane of cellulose, the cell-wall determines + ~ 
= the size and form of the cell. Although cells vary relatively er 
~).. so greatly in size, they are generally very minute even in- 
| - their mature state, being scarcely visible to the naked eye, or 
-‘ altogether invisible. The unicellular Zo0glea Termo tor 
Fic. 1o.—Tabular cell 
- . from the epidermis of 
. Callitriche. 
Fic. 11.—Irregularly stellate cell: a uni- 
Vg ae cellular Alga, Euastrume crux-neliten- 
AS - §ts. (X 400.) . ‘ 
| example, is from o‘oor to o’oo2 mm. broad and long; 
-' the diameter of pollen-grains, which also consist of a single 
+ cell, varies between ovr and 00075 mm. ; the lignified cells 
in the wood of the oak (Quercus pedicellata) have an average ~ 
| | length of about o-7 mm., and those in the trunk of the silver 
fir, Abzes pectinata, one of from 2 to 3mm. Thereare, how- 
ein S: ever, cells of a considerably larger size, especially in. -— 
the Characez ; thus /Vi7e//a has cylindrical cells more than 50 
| mm. long, and 1 mm. in diameter. The walls which bound 
‘the cells are seldom flat, but usually curved, of perfectly — | 
spherical, cylindrical, or even branched and arborescent form. —_- 
The most common forms of cells are—spherical (Fig. 4); 
