THE MORPHOLOGICAL COMPOSITION OF PLANTS. 51 



ence exhibited between closely allied genera of one group, the 

 Mosses. Equally varied are the developments of the foliar- 

 organs in their detailed structures: now being without mid- 

 ribs or veins; now having mid-ribs but no veins; now having 

 both mid-ribs and veins. Nor must we omit the similarly-sig- 

 nificant circumstance, that whereas in the lower Archegoniates 

 the reproductive elements are immersed here and there in the 

 thallus-like frond, they are, in the higher orders, seated in 

 well-specialized and quite distinct fructifying organs, having 

 analogies with the flowers of Phsenogams. Thus, many facts 

 imply that if the Phasnogamic type is to be analyzed at all, 

 we must look among the Archegoniates for its morphological 

 components, and the manner of their integration. 



Already we have seen among the lower Cryptogamia, how, 

 as they became integrated and definitely limited, aggregates 

 acquire the habit of budding out other aggregates, on reach- 

 ing certain stages of growth. Cells produce other cells 

 endogenously or exogenously; and fronds give origin to 

 other fronds from their edges or surfaces. We have seen, too, 

 that the new aggregates so produced, whether of the first 

 order or the second order, may either separate or remain 

 connected. Fissiparously-multiplying cells in some cases 

 part company, while in other cases they unite into threads or 

 laminae or masses; and fronds originating proliferously from 

 other fronds, sometimes when mature disconnect themselves 

 from their parents, and sometimes continue attached to them. 

 Whether they do or do not part, is clearly determined by 

 their nutrition. If the conditions are such that they can 

 severally thrive better by separating after a certain develop- 

 ment is reached, it will become their habit then to separate; 

 since natural selection will favour the propagation of those 

 which separate most nearly at that time. If, conversely, it 

 profits the species for the cells or fronds to continue longer 

 attached, which it can only do if their growths and subse- 

 quent powers of multiplication are thereby increased, it must 

 happen, through the continual survival of the fittest, that 



