270 MICROSCOPIC FORMS OF VEGETABLE LIFE, 



division occurs) occupying the intermediate place. At last the 

 central fusion becomes complete, and two bipartite fronds are 

 formed, each having one old and one young segment ; the young 

 segment, however, soon acquires the full size and characteristic 

 aspect of the old one ; and the same process, the whole of which 

 may take place within twenty-four hours,' is repeated ere long. 

 In Sphcerozosma, the cells thus produced remain connected in 

 rows within a gelatinous sheath, like those of Didymoprium 

 (Fig. 77) ; and different stages of the process may commonly be 

 observed in the different parts of any one of the filaments thus 

 formed. In any such filament, it is obvious that the two oldest 

 segments are found at its opposite extremities, and that each 

 subdivision of the intermediate cells must carry them further 

 and further from each other. This is a very different mode of 

 increase from that of the Confervaeece, in which the terminal cell 

 alone undergoes subdivision (§ 198), and is consequently the last 

 formed. 



166. Many of the DesmidiacecR multiply after another method; 

 namely, by the subdivision of their endochrome into a multitude 

 of granular particles, termed gonidia ; which are set free by the 

 rupture of the cell-wall, and of which every one may develope 

 itself into a new cell. These "gonidia" may be endowed with 

 cilia, and may possess an active power of locomotion, in which 

 case they are known as "zoospores;" or they may be destitute 

 of any such power, and may become enclosed in a firm cyst or 

 envelope that seems destined for their long-continued preserva- 

 tion, in Avhich case they are designated as " resting-spores." The 

 movement of the zoospores, first within the cavity of the cell 

 that gives origin to them, and afterwards externally to it, has 

 frequently been observed in the various species of Oosmarium; 

 and has been described under the title of " the swarming of the 

 granules," from the extraordinaiy resemblance which the mass 

 of moving particles bears to a swarm of bees. The subsequent 

 history of their development, however, has not been fully traced 

 out; and this is a point to which the attention of Microscopists 

 should be specially directed. In Pediastrum, a plant whose frond 

 normally consists of a cluster of cells, the zoospores are not 

 emitted separately, but those formed by the subdivision of the 

 endochrome of one cell, which may be 4, 8, 16, 32, or 64 in 

 number, escape from the parent frond still enclosed in the inner 

 tunic of the cell ; and it is within this that they develope them- 

 selves into a cluster, resembling that in which they originated. 

 This is well shown in the accompanying series of illustrations of 

 the developmental history o? Pediastrum granulatum (Fig. 73, a-f), 

 a species whose frond normally consists of 16 cells, but may be 



' See the observations of Mrs. Herbert Thomas on Cosmarium mar^aritiferum, in 

 "Microscopical Transactions," Second Series, vol, iii, pp. 33-36. Several varieties in 

 the mode of subdivision are described in this short record of long-continued observa- 

 tions, as of occasional occurrence. 



