SEAWEEDS AND THE WATER-FLORA. 1 79 



torn away by currents in the water. Some Desmids, 

 when they are in danger of being carried away, are able 

 to double themselves round, and hold on to any con- 

 venient support. They multiply like diatoms by a 

 process of division, or by conjugation. Occasionally, 

 two of these Desmids will unite to form a hard-walled 

 spore or egg-cell, which, after a season of rest, grows 

 and multiplies. 



A very remarkable free-swimming Alga, Volvox, 

 consists of a hollow sphere of cells, held together by 

 transparent mucilage. Each cell has two thin whips, 

 or cilia of protoplasm, and these, by vigorously lashing 

 the water, make the colony revolve, so that it moves 

 quickly through the water, tumbling over and over as it 

 does so. The colony reminds one of the blastosphere of 

 the animal world, but plants have not developed further 

 than Volvox in this direction, for it is quite unlike any 

 other Alga. In the sea, large free-swimming plants are 

 not common ; the Sargasso "Weed which covers the Sar- 

 gasso sea {^Cystophylliun or Sargassuui) is a brown Alga. 

 It is remarkable for the small buoys or bladders which 

 assist in keeping it floating. A few Ferns, such as 

 Salvinia, or Liverworts {Riccia\ and some flower- 

 ing plants, are also quite independent of the earth, 

 and float on the surface of fresh-water lochs and rivers. 

 The curious little Duckweed {Lenma) has two minute 

 fleshy fronds about a quarter of an inch long, with 

 hanging roots attached to them. This plant occurs in 

 almost every pond. The Water-soldier, Stratiotes, 

 resembles a small lettuce head, and is especially abun- 

 dant upon the Nile. The Florida Hyacinth, which occurs 

 in the St. John's river and other places, floats by means 

 of its swollen leaf stalks, which prevent it from being 

 upset (see p. 57). 



