804 



THE DISTRIBUTION OF SPECIES BY OFFSHOOTS. 



which the offshoot formation occurs is divided into four, more rarely two or eight 

 clumps are formed by the splitting and dividing of the protoplasm, and most rarely 

 of all is the whole undivided protoplasm of a cell transformed into a single offshoot. 

 The small group of water-plants known as the Hydrodictyacese display a peculiar 

 offshoot-formation. In the elegant Water-net (Hydrodictyon utriculatum, cf. 

 p. 640), whose cylindrical cells form a closed net with hexagonal meshes, the cells 

 each originate new plants as Water-nets in miniature. The protoplasm in one of 



Fig. 447.— Frogbit (Hydroeharis Mm'sus-ranw). The winter buds in process of detachment from the ends of the submerged 



stolons. 



the cells which is preparing for offshoot-formation divides into many thousand parts 

 which quiver in a remarkable way and pass between one another, and are said to 

 undergo the so-called swarming motion. This lasts about half an hour, then the 

 swarming portions, whose rod-like form can be recognized in spite of their minute- 

 ness, come to rest, arrange themselves into nets with hexagonal meshes (see 

 figs. 370 ^•*'^' p. 640), and now each cell contains a tiny Water-net. The outer layer 

 of the cell-wall in which this grouping has taken place is partially dissolved. The 

 little net, at first still inclosed in a pellicle of protoplasm, slips out and swims freely 

 in the water as an offshoot. In 3-4 weeks it has attained the size of the Water-net, 

 from one of whose cells it emerged, and in each of its own cylindrical cells the same 

 process may be repeated. A similar process is observed in the small water-plant 



