FRESH-WATER ALGJC. 35 



2. — Zygospores, in which two similar cells coalesce and 

 produce a reproductive cell termed a zygospore, which germinates 

 after a longer or shorter period of rest, and gives rise either to 

 spores, or to a plant of the same kind as that in which conjuga- 

 tion took place. 



3. — Oospore.^, in which the two reproductive cells are essen- 

 tially different. The female cell, or Oosphere, is enclosed within 

 an older cell, the Oogonium. The male cells, the Antherozoids, 

 are enclosed within a larger cell, the Antheridium, and are 

 endowed with motion by means of vibratile cilia. They swarm 

 round the Oosphere and cause its impregnation by the coalescence 

 of their substance with it ; the Oosphere becomes invested with a 

 thick cell wall, and is now called the Oospore. After a longer or 

 shorter period of rest, the Oospore germinates, and gives rise 

 either to a plant resembling the mother plant, or to a number of 

 Zoospores, each of which finally gives rise to a plant like the 

 mother plant. 



4. — Carpospores, in which there is a general resemblance to 

 Oosporeae, but a great deal more individual variety. The female 

 element consists of one or more cells, and is called the Carpogo- 

 nium. The male cell varies greatly, consisting of antherozoids 

 either swarming or passively motile, or of tubular pollinodia. 

 Fertilisation is effected by the entrance of antherozoids into the 

 female cell as in the Oosporeae, and the product is called a Spo- 

 rocarp. This is sometimes a single cell germinating directly, or 

 through the medium of zoospores, or more generally a multicellu- 

 lar body from which spores are finally produced. 



One essential difference between Sporocarps and Oospores 

 consists in this : that in the production of the former certain cells 

 also take part which were not immediately concerned in the act of 

 impregnation ; and that the portion of the fructification producing 

 the spores is surrounded by a sterile envelope, which serves 

 merely for protection, or for further nourishment. 



There is, however, another method of propagation common to 

 all these four classes — viz., by means of Gonidia, which are pro- 

 duced quite independently of any act of fertilisation. The 

 Gonidia often arise from the Thallus by the whole of the contents 

 of certain cells dividing, and producing two or more Gonidia, 



