JAMES BURTON OX HYDRODICTYOX RETICULATUM. 589 



were separate, and sank to the bottom of the water, the} 7 would 

 be liable to become overwhelmed in the mud and debris, while 

 in their present condition they would rest on the bottom without 

 danger of being covered up. It may also be noticed that an 

 organism formed like the water-net. when it is in active life 

 under the influence of warmth and light, excretes gas, and forms 

 bubbles, which are entangled in the meshes and float the whole 

 colony to the surface where it obtains better light and purer 

 water. Another advantage may be that the separate cells, 

 being for a time very small, would be liable to be taken as food 

 by various small aquatic animals, a fate to which they are 

 much less subject when combined into a larger body. Many 

 of the filamentous algae not usually looked upon as composed 

 of individuals, as coenobia in fact, are so in reality. This is the 

 case, for instance, with the well-known Spirogyra ; here each 

 cell of the filament if separated would be able to carry on its 

 vital functions, and probably the chief advantage it gains from 

 its form is something of the kind already mentioned. 



But this brings me to the next point of interest in the water-net. 

 In Spirogyra and almost all other freshwater algae, multiplication 

 very largely takes place in many species there is no other 

 method of propagation by means of what we may call vegeta- 

 tive reproduction. A cell grows till it reaches its maximum 

 size, a wall is then formed across it and the one large mature cell 

 becomes two smaller young ones which gradually grow, and the 

 process is repeated. Now in Hydrodictyon there is no division 

 of a cell. You may examine any number of plants, each con- 

 sisting of perhaps thousands of cells, and you will never find one 

 undergoing cell-division. The cell begins quite small and grows 

 till it reaches what is a very large size for such an organism, 

 but it never gives rise in this way to another. From this a 

 singular result arises. The net is born, as we may say, with a 

 given number of cells, and through its life it consists of only the 

 same number and indeed of the identical ones which it had 

 originally. If owing to injury a part of the net is destroyed, it 

 is not replaced, the deficiency cannot be made good. 



Another unique fact is the method of reproduction ; no other 

 alga has the same in detail. In the non-sexual method which 

 is, I think, the most usual and is indeed the only kind of which 

 I have had actual experience a small complete net consisting, 



