290 PART IV. VEGETABLE TAXONOMY. 



considerably in size, their protoplasm divides into a multitude of 

 small cells, which become filled with an orange-colored endo- 

 chrome, acquire cilia, and finally also escape into the interior 

 of the colony, when they swarm about the oospheres, penetrate 

 into their protoplasm and fertilize them. After the resulting 

 oospores have matured, the walls of the parent colony dissolve 

 away and the spores are set free. Fig. 485 represents a fertile 

 colony of this species. 



The Confervaceae. The plants of this group are all chloro- 

 phyll-green algae, chiefly inhabiting fresh waters, and most of 

 them are either filamentous or they form a flattened thallus, con- 

 sisting of a single layer, or at most of two layers, of cells. In a 

 few forms the filaments are branching, in a few others, adjacent 

 filaments anastomose, and in some, the component cells secrete 

 a mucilage and become separated from each other, forming a 

 mucilaginous mass, in which the cells multiply by division. 



Most of the forms reproduce by asexual zoospores, many 

 reproduce sexually by means of conjugating zofipores, still others 

 produce oogonia containing one or more oospheres, and anthe- 

 ridia which produce ciliated antherozoids ; but there are still 

 others whose life-histories have not yet been traced out. 



The order includes a large number of forms, grouped mainly 

 under the following families : the Ulvaceae, the Ulothricacese, 

 the Spheroplese, the Oedogonieae and the Coleochgetse. 



The Ulvas, so common in marine estuaries and salt marshes, 

 are often called Sea-lettuce, from the shape of the bright-green 

 fronds, which consist of thin, flattened or crispate membranous 

 expansions, often several inches in breadth, and attached to 

 stones, shells, etc. The fronds consist of two strata of cells, and 

 increase in size by cell division in two planes. In the early 

 stages of their development, however, they are filamentous, but 

 afterward, by cell-division in two planes, become laterally ex- 

 panded. After a time they produce bi-ciliated or sometimes 

 quadri-ciliated swarm-spores. In one species at least, Ulva 

 Lactuca, some of the spores have been observed to come together 

 by their ciliated ends and fuse into an oval mass, forming a zyg- 

 ospore. This after a time divides and forms a colony of indi- 

 vidual cells, each of which is capable of developing into the 

 mature form of the plant. In this case there is no evident dis- 



