54 



THE GENUS HYDRODICTYON. 



By Dr. Horatio C. Wood, Jun.* 



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 appreciable 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 in 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 end 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 to end in such a way as to 

 form polygonal and mostly pentagonal meshes, the size of which 

 varies with the age of the plant. These cells, which are closely 

 conjoined, but have no passage-ways between them, are capable of 

 independent life, so that the Hydrodictyon may be looked upon as 

 an elaboiate type of a cell-family, one in which cells are conjoined 



* Extract from " A Contribution to the Natural History of the Fresh Water 

 Algas of North America." 1873. 



