CHAPTER V. — THE THALLOPHYTA. 285 



They reproduce asexually either by the separation of the ends 

 of some of the branches, or by the escape of swarm-spores 

 clothed with cilia. 



Their mode of sexual reproduction is illustrated in Fig. 480. 

 The oogonium, it will be seen, is an oval body borne laterally on 

 the filament, and cut off from it by a partition. The antheridal 

 branch, which is located adjacent to it, is slender and curved, a 

 transverse partition is formed near its middle, and in the terminal 

 cell thus produced are developed an immense number of very 

 minute, ciliated antherozoids, which are discharged by a rupture 

 of the cell-walls. Some of them find their way through the 

 terminal opening of an oogonium, and fertilize the contained 

 oOsphere. It then develops into an oospore, which, after resting 

 until the succeeding spring, germinates. 



The species of Bryopsis ; the Acetabularias, curious umbrella- 

 shaped marine forms ; and the Caulerpas, sea-shore plants, some- 

 times attaining a length of several yards, and having rhizoids, 

 creeping stems and leaf-like branches, looking wonderfully like 

 the much more highly organized multicellular plants, which have 

 roots, stems and leaves, are also classed with the Cceloblasteae. 



The Protococcaceae include a number of forms of fresh- 

 water algae, some of which occur as isolated cells, while others 

 consist of cells once distinct, which have united to form cell- 

 colonies. These colonies have a definite shape peculiar to the 

 species, and are called ccenobia. In the vegetative form the plants 

 are not endowed with the power of locomotion, and in this 

 respect differ from the Volvocineee. Cell-division occurs usually 

 by internal cell-formation, and the cells always at first become 

 distinct from each other, though they sometimes unite afterwards, 

 as we have seen, to form ccenobia. They reproduce asexually 

 by means of zoospores, and most of them also sexually by the 

 conjugation of zoospores of smaller size. 



Some of the unicellular forms live in the interior of other 

 plants, though not parasitically. One of these, called Chloro- 

 chytrium Lemnge, grows in the intercellular spaces of the thallus 

 of the common Duck-weed, Lemna trisulca. 



Among the forms which produce Ccenobia are the Pedias- 

 trums and Hydrodictyon. The former are free microscopic 

 forms, found abundantly in most fresh waters. The shapes of 



