652 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



Sphceropleacece. — Sphceroplea annulina is a curious Alga which appears occa- 

 sionally on flooded fields or in other situations. Its thallus consists of simple 

 threads of cells which are of very various length, sometimes enormously long. The 

 side walls are thin, but the transverse walls are often thick, and both are liable to 

 have curious thick and quite irregular projections of cellulose. The chromatophores 

 form irregular rings at intervals, and contain many pyrenoids. There are man}' 

 nuclei, and several variable vacuoles in each cell. All the cells may produce sexual 

 organs, the threads being either monoecious or dioecious. The contents of the cells 

 which become antheridia become yellow-red, and break up to form a great number 

 of elongated, pointed spermatozoids. These escape through small holes in the wall. 

 In the oogonia the protoplasm divides to form one or two series of spherical 

 oospheres, each with a colourless spot. The oospore has three membranes, of which 

 the outermost is folded so as to give the oospore a star-like appearance. 



In germination the oospore produces 1-8 zoospores, which have a green posterior 

 and a pale-red anterior end. Each eventually stretches itself to form a spindle- 

 shaped cell, and a multiplication of nuclei and pyrenoids takes place before trans- 

 verse divisions occur and a new Sphceroplea-ü\a,ment is produced. Parthenogenesis 

 occurs, but apparently no zoospores are formed other than those produced in the 

 germination of the oospore. 



Chcetophoracece. — This family contains forms with a branching thallus, the 

 branches often ending in fine hairs. The chromatophore is parietal, with one or 

 more pyrenoids. Zoospores with two or four, and gametes with two flagella are 

 produced. 



Stigeoclonium, Draparnaldia, and Chcetophora are three genera common in fresh- 

 water in this country, and all very slimy to the touch. In the first-named genus the 

 thallus is fixed by means of a basal disc of cells, the sole; the branching is simple 

 and irregular, the branches often ending in long multicellular hairs. Draparnaldia 

 shows a marked distinction between axis and appendages. The axial cells are 

 much larger, and at the same time poorer in chlorophyll than those of the branches. 

 The branches come off in bunches, and often end in many-celled hairs. The chro- 

 matophore possesses many pyrenoids varying in number according to the size of 

 the cell. Chwtophora possesses a thallus whose threads radiate and branch in all 

 directions. The whole is surrounded by a mucilaginous investment of considerable 

 firmness, sometimes almost leathery in consistence, so that a Chcetopho ra-plant has 

 the appearance of a slimy green ball. 



There are several other genera belonging to this family, many of them being 

 epiphytic or endophytic. Entoderma lives in the cell-membranes of the Brown 

 Sea-weed Ectocarpus. 



Trentepohliaceos. — This family differs from the last in possessing no hairs, and in 

 forming its zoospores in special zoosporangia. 



Trentepohlia is a fairly large genus containing forms mostly living in the air 

 on damp stones and similar situations. The thallus consists of rounded thick-walled 

 cells, and is dichotomously or irregularly branched, partly creeping, partly upright. 



