SIZES OF CELLS. 41 
illustrations of cells which are more or less unrestrained in 
their development (figs. 137-143) ; other instances occur in the 
germination of most spores, and strikingly so in those of many 
Alge, as Botrydiwm (jig. 78); also when the pollen cells fall 
upon the stigma ; and in numerous other cases. 
2. Sizes of Cells.—The cells vary much in size in different 
plants, and in different parts of the same plant. The parenchy; 
matous cells, on an average, vary from about 535 to ygo5 Of an 
inch in diameter ; others again are not more than 35,3; while 
in some cases they are so large as to be visible to the naked eye, 
being as much as 3, or even 5 of an inch in diameter. The 
largest occur in the pith of plants, in succulent parts, and in water 
‘plants. 
: The dimensions of prosenchymatous cells generally afford a 
Fig. 73. Fie. 75. Fia. 76. 
Oo \ 
Pre. 74. Bie. 77. 
Fig. 73. Spherical pollen cell with small projections or papille on its outer 
surface.——/ig. 74. Cylindrical cell covered with warty projections.—— 
Figs. ie Ciliated cells. Fig. 78. Branched cell (Botrydium granu- 
datum). 
striking contrast to those of the parenchymatous, for while we 
find that their transverse diameter is commonly much less, 
averaging about ;2;, of an inch, and frequently not more than 3,55; 
they become much more extended longitudinally, some having 
been measured as much as } of an inch or more long, and ac- 
cording to Schleiden, those of the inner bark are often four, five, 
or more inches in length. The prosenchymatous cells of the wood 
and inner bark of trees generally vary, however, from about the 
35 to the ;5 of an inch in length. 
Those cells again which have an unrestrained development 
are frequently also far more extended in length. Thus, the cell 
of which each filament of cotton is formed (fig. 157, a) is 
sometimes as much as one or two inches long ; while in some of 
