72 



POPULAR SCIENCE MONTHLY. 



with great ease in the laboratory. This plant is a great cell colony, 

 involving usually thousands of elements which are joined to one an- 

 other to form a net of polygonal meshes. A portion of such a plant 

 is shown in Figure 3, a. These cells after they have reached a certain 

 age produce zoospores that may be either asexual or sexual (gametes). 

 The gametes are smaller and are produced much more numerously than 



Fig. 3. Hydrodictyon. a, Portion of Net. 6, End of Cell showing Young Net in 

 ITS Interior, c, Gahete. d, e, Conjugation of Gametes. 



the asexual elements. They escape from the mother-cell and after 

 swarming in the water conjugate in pairs (see Figure 3, c, d, e). The 

 asexual swarm-spores have the peculiarity of never leaving the mother 

 cell. They swim around in the cavity bounded by the cell wall and 

 shortly come to rest, arranging themselves to form a new net entirely 

 within the parent cell. Thus by their habits one may readily distin- 

 guish the asexual zoospores and gametes of Hydrodictyon. 



Now let us summarize the factors that will make Hydrodictyon 

 form asexual zoospores at one time and gametes at another. The water- 

 net grows luxuriantly in a culture solution containing a number of 

 inorganic salts. If plants are removed from such a culture solution 

 and placed in fresh water they will develop zoospores in 34 hours. The 

 process takes place most rapidly if the temperature is slightly above 

 the normal; indeed merely warming the water in which plants are 

 living will frequently induce the production of zoospores. Plants re- 

 fuse to form zoospores at a temperature as low as 8° C. but if such a 

 culture be raised to the warmth of 16° or 20° the process is immediately 

 resumed. Gametes are produced under very different conditions from 

 those stated above. They demand organic food. Cultures of Hydro- 

 dictyon in a solution of 'cane sugar will almost certainly yield gametes 

 after several days. 



It frequently happens that the nets of Hydrodictyon will exhibit 

 a well-defined tendency or preference to form either gametes or zoo- 

 spores. Such a habit may be quite thoroughly broken by cultivating 

 a plant under proper surroundings. Gamete-forming nets will shortly 



