BEITISH MOSSES. 15 



veil of the capsule; it gradually swells at the upper 

 end till it produces the capsule. In this capsule, without 

 any fertilization or union of cells, is produced a mass of 

 spores of the kind with which we began our life-history. 

 We have now completed the second part of this history, and 

 have come round to the spore with which we started. The 

 plant has thus began with the spore, an asexual cell, reached 

 the point where its whole future is gathered up in a sexual 

 cell, which has produced an organism again producing an 

 asexual cell. We started with a spore, and have returned 

 to a spore ; we have travelled round a circle, divisible into 

 two parts or generations, one sexual, the other asexual, and 

 we have therefore a case of alternation of generations. To 

 make this statement more clear, it may be observed that a 

 generation is here spoken of as that part of the life of an 

 organism which intervenes between the two points at which 

 its whole future is gathered up into one cell ; that such a 

 cell is sexual when it is the result of the combination of two 

 previously existing and independent cells ; that such a cell 

 is asexual when it is not the result of such combination ; 

 that an alternation of generations exists whenever in the 

 complete cycle of existence or life-history there are two 

 points at which the whole organism is reduced to a single 

 cell, and when the forms of the organism in the two 

 intervals of its development are different. In the Mosses, 

 where the sporogone co-exists with and is organically con- 

 nected with what I have called the Moss plant, it is evident 

 that the two generations are not such, according to the 

 more popular notion of that word they are not independent 

 nor necessarily successive. 



