REPRODUCTION. 627 



or asexual reproductive cells. Thus in Acetabularia, and 

 under certain circumstances in Botrydium, the asexually 

 produced resting-spore constitutes a gametangium, in that it 

 gives rise to a number of planogametes ; similarly the asex- 

 ually produced spore of Protomyces produces a number of 

 conjugating sporidia. In some Peronosporeae (always in 

 Cystopus, occasionally, according to circumstances, in Pythium, 

 Phytophthora, and Peronospora), the asexually produced 

 spore behaves as a sporangium, and gives rise to a number 

 of zoospores from each of which a new individual is 

 developed. 



The same thing happens occasionally also in the case of 

 sexually produced spores. Among the Fungi, the formation 

 of zoospores in the oospore occurs in various species of Pero- 

 nosporeae and Saprolegnieae. Among the Algae, zoospores are 

 formed in the zygospores of Pandorina and Ulothrix, and in 

 the oospores of Oedogonium and Sphaeroplea. Cases of a 

 similar kind are known in Phanerogams. Thus, in some 

 Coniferai, and notably in the Gnetaceous Ephedra altissima, a 

 process of cell-formation goes on in the oospore, leading to the 

 formation of a larger or smaller number of cells from each of 

 which an embryo-plant is developed. All these cases in which 

 the spore, whether sexually or asexually produced, gives rise 

 to a number of cells, each of which is capable, by itself, of 

 developing into a new individual, are instances of what is 

 known as polyembryony . 



In some cases free cells are formed in the reproductive cell which are 

 not reproductive but somatic; this obtains in the Hydrodictyeae. In 

 Hydrodictyon utriculatum, the protoplasm of the zygospore gives rise to 

 two or four large zoospores which eventually come to rest and remain 

 quiescent for several months ; these resting-spores are termed, on account 

 of their form, polyhedra. On germination the protoplasm of the poly- 

 hedron breaks up into a number of small ciliated motile cells, the 

 endospore protruding as a delicate vesicle within which the motile cells 

 are in active movement. The motile cells eventually come to rest, without 

 escaping from the endospore, and arrange themselves so as to form the 

 meshes of a small sac-like net which is a young Hydrodictyon. The 

 endospore is then disorganised, and the young net is set free as an 

 independent ccenobium, 



4O 2 



