PROTOCOCCACE^E. 35 



The commonest of British species. A variety has been described 

 which differs only in being entirely destitute of bristles. We can con- 

 firm Ralfs in his observation that the species of this genns frequently 

 make their appearance in clear water that is kept in glasses or bottles 

 and exposed to the light. He says that he has repeatedly noticed the 

 appearance of 8. acutus var. obliquus in bottles containing Desmidiece, and 

 sometimes its rapid increase so as to outnumber its companions. Speci- 

 mens obtained in this manner, he adds, are frequently more or less 

 distorted. In little aquaria the present species often becomes a nuisance 

 from its profusion. 



Plate XIII. fig. 8. Cells magnified 400 diameters. 



Sub-Family 5. HYDRODICTYE^. 



Individual cells oblong-cylindrical, united into a reticu- 

 lated saccate coenobium, all fertile, some producing macro- 

 gonidia, which join themselves into a coenobium within the 

 mother- cell, others producing microgonidia, which are fur- 

 nished with two vibratile cilia and a lateral red spot ; these 

 escape from the parent-cell, and, after a brief motile period, 

 subside into protococcoid, thick-walled spores. 



GENUS 26. HYDRODICTYON. Roth. (1800.) 



Characters the same as in the sub-family. 



" The genus Hydrodictyon comprises, as far as known, but a single 

 species, which is common to North America and Europe. It grows in 

 great abundance in the neighbourhood of Philadelphia, especially in the 

 ditches and stagnant brick ponds in the low grounds below the city, 

 known as the ' Neck.' There it very frequently forms floating masses 

 several inches in thickness, and many feet in extent, so that with the aid 

 of a rake it could be gathered by the bushel. When thus in mass the 

 colour is very generally dingy and yellowish, although the fronds, when 

 in active vegetative life, are mostly of a bright, beautiful green. The 

 plant is in greatest profusion in June and July, after which time it 

 gradually disappears, until in the autumn it is scarcely to be found, but 

 early in the spring it reappears. The very young fronds are minute, 

 oval, cylindrical, filmy. looking closed nets, with the meshes not appre- 

 ciable to the eye ; when growth takes place the fronds enlarge, until 

 finally they form beautiful cylindrical nets, two to six inches in length, 

 with their meshes very distinct, and their ends closed. In the bright 

 sunlight, they, of course, by virtue of the life functions of their chlorophyl, 

 liberate oxygen, which, being free to the interior of the net, and its exit 

 barred by the fine meshes, collects as a bubble in one end of the cylinder, 

 and buoys it up, so that, the heavier ends sinking, the net is suspended, 

 as it were, vertically in the water. I know of few things of the kind 

 more beautiful than a jar of limpid water with masses of these little nets 

 hanging from the surface like curtains of sheen in the bright sunlight. 

 A few cells collected in the fall or early spring, if put into a preserving 

 jar, and the water occasionally changed, will multiply, and in a little while 

 become a source of frequent pleasure to the watcher. 



" As the fronds increase in size they are always in some way or other 

 broken up, so that, instead of being closed cylinders, they appear as 

 simple open networks of less or greater extent. The extreme length to 

 which the frond attains is, I think, very rarely over twelve inches, with 

 meshes of about a third of an inch in length. The construction of the 

 frond is always the same. It is composed of cylindrical cells united end 



