GROWTH A.ND ASEXUAL GENESIS. 421 



that 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 Volvocince this continual dissolution of a primary 

 compound individual into secondary compound individuals, is 

 carried on endogenously — the parent bursting to liberate tluj 

 young ; and the numbers arising by this method, also are some- 

 times so o-reat as to tint laro-e bodies of water. More 

 fully established and organized aggregates of the second 

 order, such as the higher Thallogens and the lower Acrogens, 

 do not sacrifice their individualities by fission ; but never- 

 theless, by the kindred process of gemmation, are continually 

 hindered in the increase of their individualities. The gemmae 

 called tetraspores are cast ofi" in great numbers by the marine 

 AlgcB. Among those simple Jungennanniacece which consist 

 of single fronds, the young ones that bud out grow for a time 

 in connexion vith 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. Acrogens 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 roct, 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 

 ver}^ unusual among Phaenogams. The detachment of bulbils, 

 though not unknown among them, is exceptional. And while 

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

 multiply by a process ained to gemmation, yet this is 

 anything but characteristic of the class. A leading trait ot 



