FRESH-WATER ALG^ OF THE UNITED STATES. 93 



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 elaborate 

 type of a cell-family, one in which cells are conjoined in accordance with a defi- 

 nite plan, so as to make a body of definite shape and size, yet in which each cell 

 is an independent being, drawing nothing from its neighbors. The cells them- 

 selves are cylindrical, with a thickish cellulose wall, and have no nuclei. Their 

 chlorophyllous protoplasm is granular, and is placed in the exterior portion of the 

 ceil, forming thus, within the outer wall, a hollow cylinder, in which are imbedded 

 starch granules, and whose interior is occupied with watery contents. The hydro- 

 dictyon cell, when once Jbrmed, is capable of growth, but not of going through the 

 usual process of cell multiplication by division, so that the adult frond is com- 

 posed of just as many and indeed the same cells, as it had in its earliest infancy. 



No true sexual reproduction has as yet been discovered in the water-nets. There 

 have been described, however, two forms or methods in which the species multi- 

 plies, both of them occurring by means of motile zoosporoid bodies. In the one 

 case these develop immediately into .the new plant, whilst in the other before 

 doing so they pass through a resting stage. Of the life-history of the latter, the 

 microgonidia, I have no personal knowledge. 



The investigation of the production and development of the macrogonidia, how- 

 ever, has occupied considerable of the time devoted by myself to the microscope, 

 and I have seen large numbers of specimens in almost aU the stages of develop- 

 ment, i have never been able to detect, however, any decided motion in the 

 macrogonidia. 



They are formed in the protoplasmic stratum, already alluded to as occupying 

 the outer portion of the interior of the hydrodictyon cell. The first alteration in 

 this, presaging their formation, is a disappearance of the starch granules, and a 

 loss of the beautiful, transparent green color. Shortly after this, even before all 

 traces of the starch-grain are gone, there appear in the protoplasm numerous 

 bright spots placed at regular intervals; these are the centres of development 

 around which the new bodies are to form. As the process goes on, the chlorophyl 

 granules draw more and more closely around these points, and at the same time 

 the mass becomes more and more opaque, dull, and yellowish-brown in color. This 

 condensation continues until at last the little masses are resolved into dark hexa- 

 gonal or polygonal plates, distinctly separated by light, sharply defined lines. In 

 some, the original bright central spot is still perceptible, but in others it is entirely 

 obscured by the dark crowded chlorophyl. The separation of these plates now 

 becomes more and more positive, and they begin to become convex, then lenticular, 

 and are at last converted into free, oval, or globular bodies. When these are fully 

 formed, they are said to exhibit a peculiar trembling motion, mutually crowding 

 and pushing one another, compared by M. Braun to the restless, uneasy movement 

 jseen in a dense crowd of people in which no one is able to leave his place. Whilst 

 the process just described has been going on, the outer cellulose wall of the hydro- 

 dictyon cell has been undergoing changes, becoming thicker and softer and more 



