58 MORPHOLOGICAL DEVELOPMENT. 



and this mechanical advantage. And from the particular 

 class of plants we are here dealing with the Archegoniates 

 a type is shown in Fig. 78, Riella helicophylla, similarly char- 

 acterized by a thin frond that is made stiff enough to stand, 

 by an incurving which, though it does not produce a hollow 

 cylinder, produces a kindred form. If, then, as we have 

 seen, natural selection or survival of the fittest will favour 

 such among these recumbent Archegoniates as are enabled, 

 by variations in their structures, to maintain raised postures; 

 it will favour the formation of fronds that curve round upon 

 themselves, and curve round upon the fronds growing out 

 of them. What, now, will be the result should such a 

 modification take place in the group of proliferous fronds 

 represented in Fig. 76? Clearly, the result will be a 

 structure like that shown in Fig. 79. And if this inrolling 

 becomes more complete, a form like Jungermannia cordifolia, 



represented in Fig. 80, will 

 be produced. 



When the successive 

 fronds are thus folded round 

 so completely that their 

 opposite edges meet, these 

 opposite edges will be apt to 

 unite: not that they will 

 grow together after being 

 formed, but that they will 

 79 80 develop in connexion; or, 



in botanical language, will become " adnate." That foliar 

 surfaces which, in their embryonic state, are in close contact, 

 often join into one, is a familiar fact. It is habitually so 

 with sepals or divisions of the calyx. In all campanulate 

 flowers it is so with petals. And in some tribes of plants 

 it is so with stamens. We are therefore well warranted in 

 inferring that, under the conditions above described, the suc- 

 cessive fronds or leaflets will, by union of their remote edges, 

 first at their points of origin and afterwards higher up, 



