GROWTH AND ASEXUAL GENESIS 441 



thai under favourable conditions a single colony may on the 

 second day develop 16, on the third 256, on the fourth 4,096, 

 and at the end of a week 268,435,456 other organisms like 

 itself." In the Volvocinece this continual dissolution of a 

 primary compound individual into secondary compound indi- 

 viduals, is carried on endogenously, and on a modified system: 

 some only of the component cells giving origin to young 

 colonies, and the parent bursting to liberate them. The 

 numbers arising by this method also, are sometimes so great 

 as to tint large bodies of water. More fully estab- 



lished and organized aggregates of the second order, such as 

 the higher Thallophytes and the lower Archegoniates, do not 

 sacrifice their individualities by fission; but nevertheless, by 

 the kindred process of gemmation, are continually hindered 

 in the increase of their individualities. The gemmae called 

 tetraspores are cast off in great numbers by the marine Algce. 

 Among those simple Jungermanniacece which consist of single 

 fronds, the young ones that bud out grow for a time in con- 

 nexion with their parents, send rootlets from their under 

 sides into the soil, and presently separate themselves — a 

 habit which augments the number of individuals in propor- 

 tion as it checks their growths. 



Plants of the third order of composition, arising by arrest 

 of this separation, exhibit a further corresponding decrease 

 in the abundance of the aggregates formed. Archegoniates of 

 inferior types, in which the axes produced by integration of 

 fronds are but small and feeble, are characterized by the 

 habit of throwing off bulbils — bud-shaped axes which, falling 

 and taking root, add to the number of distinct individuals. 

 This agamic multiplication, very general among the Mosses 

 and their kindred, and not uncommon under a modified form 

 in such higher types as the Ferns, many of which produce 

 young ones from the surfaces of their fronds, becomes very 

 unusual among Phamogams. The detachment of bulbils, 

 though not unknown among them, is exceptional. And while 

 it is true that some flowering plants, as the Strawberry, 



