286 



PART IV. VEGETABLE TAXONOMY. 



Fig. 481. — Pediastrium 

 Boryanum, magnified 350 

 diamters. 



the coenobia are roundish or stellately discoidal. Usually the 

 central cells of the disc are polygonal, and those constituting 

 the outer circle are commonly two-lobed. 



The contents of the cells after a time break up into small 

 rounded masses, which acquire cilia, and move about for a time 

 on the interior of the cell. They then break through the mem- 

 brane and escape ; they soon come to rest, however, and divide 

 again and again to form a colony, which at first consists of 

 loosely and irregularly aggregated masses of cells. The colony 

 now becomes enclosed in a mass of jelly 

 secreted by its members, and then the latter 

 arrange themselves in one plane to form such 

 a coenobium as that already described. Some 

 of the cells of the coenobia have also been 

 observed to produce another kind of ciliated 

 spore much more minute. There is reason 

 to believe that these conjugate, but the 

 process has not been actually observed. Fig. 

 481 represents the mature coenobium of one of these plants. 



The Hydrodicyon, or Water-net, is an interesting fresh- 

 water alga, not uncommon in ponds, lakes and slow streams. In 

 the mature form of the plant the cells are arranged to form a 

 quite regular net-work, which takes the shape of an elongated 

 purse or bag, often attaining a length of eight or ten inches. 

 Asexual reproduction takes place as follows : In the interior of 

 some of the cells composing the mature net, the protoplasm 

 breaks up and forms a multitude of ciliated spores, sometimes 

 as many as 15,000 to 20,000. These move about actively for a 

 time within the parent cell-walls, and then arrange themselves to 

 form minute new nets, which are finally set free by the rupture 

 or solution of the enclosing walls, and in the course of a few 

 weeks attain a size similar to that of the parent colony. 



In the sexual mode, numerous similar, but very much smaller 

 ciliated spores, are formed in some of the mature cells of the 

 colony ; these, instead of arranging themselves to form a retic- 

 ulum, escape through the mother cell-wall, conjugate in pairs, 

 and after moving about for a time, become quiescent, acquire a 

 thick cell-wall, and after a period of rest, germinate. 



Fig. 482, a, represents a portion of a mature Water-net not 



