30 INTRODUCTION. 



this mode of growth likewise takes place in all other Con- 

 ferva consisting of a single series of cells, are little less con- 

 clusive than those just enumerated. In most of the filaments 

 of these, the cells will be observed to be of various lengths, 

 some twice as long as others, and others again of every inter- 

 mediate length. Now, by means of this law of growth, the 

 variation in the length of the cells is at once and satisfactorily 

 accounted for, Avhich is not to be done in any other way. 

 But this is not all ; the progress of the formation of the 

 septa which divide the cells may be frequently traced, 

 a contraction of the cuticle and a division of the endochrome 

 gradually occurring, which is alone sufficient to establish the 

 reality of this law of increase or multiplication of cells in all 

 the true Conferva, and which may be stated to extend 

 likewise to all the other Alga the Ulvacecc, Desmidea, and 

 DiatomaceoK. In those Desmidea however which are not 

 filamentous, but which are formed of two symmetrical cells, 

 the multiplication by growth is often very different. On the 

 separation of the cells from each other, each will throw out a 

 mass of viscid mucous matter, which will go on increasing 

 until it finally takes on all the characters of the primitive 

 cell. Now, particular stress should be laid on this law of 

 developement, since it is evidently very important, inasmuch 

 as it not merely so satisfactorily and so beautifully accounts 

 for the rapid growth of all articulated Alga for it is simulta- 

 neously in operation in each of the many hundred cells of which 

 each filament is usually composed but it teaches us like- 

 wise that much caution is requisite in employing the character 

 of the length of the cells for determining species, as it proves 

 that this character, which used formerly to be much relied 

 on for the purpose, is one subject to very great variation. 

 There is a limit, however, to this law of developement which 

 does not in the genus Zygnema allow of more than one or 

 two divisions of each cell, unless indeed the spiral tubes grow 

 likewise in an equal ratio, which may be the case, and then the 

 division of the cells may be frequently repeated. In those 

 Conferva which do not contain spiral tubes the multiplication 

 of the cells may go on to an almost endless extent. 



