Flotation 245 



where produced, or accumulate in special containers. 

 These greatly reduce the specific gravity of the body, 

 enabling even heavy shelled forms (see p. 159) to float. 



3. Gelatinous and mucilaginous products of the body 

 which usually form external envelopes (see fig. 10 on 

 p. 52) but which may appear as watery swellings of the 

 tissues. Their occurrence as envelopes is very common 

 with plants and with the eggs of aquatic animals ; they 

 may serve also for protection and defense, and for 

 regulating osmotic pressure, but by reason of their low 

 specific gravity they also serve for flotation. 



Improvment of form We have already called atten- 

 tion (p. 42) to the fact that size has much to do with the 

 rate of sinking in still water. This is because the 

 resistance of the water comes from surface friction and 

 the smaller the body the greater the ratio of its 

 surface to its mass. Given a body small enough, its 

 mere minuteness will keep it long afloat. But in 

 bodies of larger size relative increase in surface is 

 brought about in various ways: 



1 . By extension of the cell in slender prolongations 

 (see fig. 50, j, k, 1, on p. 129). 



2 . By the aggregation of cells into expanded colonies : 



a. Discoid colonies, as in Pediastrum (fig. 44 on 

 p. 123). 



b. Filaments, as in Oscillatoria (fig. 34 on p. 109). 



c. Flat ribbons of innumerable slender cells placed 

 side by side, as in many lake diatoms (Fragil- 

 laria, Tabelaria, Diatoma). 



d. Radiate colonies as in Asterionella (fig. 35 n on 

 p. in). 



e. Spherical colonies as in Volvox (fig. 31, p. 105: 

 see also a b c of. fig. 50 on p. 129), wherein the 

 cells are peripheral and widely separated the 



