42 THWAITES'S THEORY. [BOOK i. 



such is really not the fact. There are some species of Pal- 

 mellese which show the character of this mucus very clearly, 

 and in which its development can be traced without diffi- 

 culty." 



" In Coccochloris cystifera may be readily observed the 

 circumstances under which the mucus is developed, and that 

 this mucus is of definite form and quantity. This, like most 

 if not all Palmellese, increases not only by an enlargement of 

 its cells, and the ordinary reproduction of these from a parent 

 cell or spore, but during the development of the plant the 

 number of cells is much increased by fissiparous division, 

 each cell becoming divided into two or four, no doubt in the 

 same way as occurs in Zygnema, Isthmia, &c. Now if the 

 plant in which this process is going onbe carefully examined, it 

 will be seen that the mucus is developed in definite quantity 

 around each cell, and doubtless by it. For we may perceive 

 one cell in which there is no indication of fissiparous division ; 

 another in which this process has just taken place, but the 

 cells are yet in close opposition ; another in which the two 

 new cells are separated to some distance from each other ; 

 and if we examine into what has led to their separation, we 

 may find that this arises from a definite development of mucus 

 around each of them, and within the mucous envelope of the 

 original cell ; and lastly, we may find a pair of new cells of 

 nearly equal size with the original one, each with nearly the 

 ordinary amount of gelatine or mucus surrounding it, and 

 the mucous sheath of the original cell nearly absorbed. In a 

 Palmella found in Sussex by Mr. Jenner, and sent me by 

 Mr. Ralfs under the name of P. hyalina, the original mucous 

 sheath appears not to be absorbed, but to be ruptured upon 

 the production of new ones within it. Each cell of some 

 species of this family is surrounded by two or more distinct 

 mucous envelopes ; and in some species a cluster of cells is 

 also surrounded by a common mucous sheath, which is no 

 doubt also developed from the cells. The curved moniliform 

 filaments of the genus Nostoc would at first sight appear to 

 grow in a mass of gelatine without any definite arrangement ; 

 but when, as is sometimes the case, the plant occurs with a 

 single straight filament, this is found to be surrounded by a 



