62 MORPHOLOGICAL DEVELOPMENT. 



result a gradual exfoliation of the successive sheaths, like 

 that indicated as beginning in the above figure of Dendro- 

 bium; which, at a, shows the bud of the undeveloped parts 

 just visible above the enwrapping sheaths, while at b, and c, 

 it shows the older sheaths in process of being split open. 

 That is to say, there must result the mode of growth which 

 helped to give the name Endogens to this class. 



The other way in which an integrated series of fronds 

 may acquire the rigidity needful for maintaining an erect 

 position, has next to be considered. If the successive fronds 

 do not acquire such habit of curling as may be taken ad- 

 vantage of by natural selection, so as to produce the requisite 

 stiffness; then, the only way in which the requisite stiffness 

 appears producible, is by the thickening and hardening of 

 the fused series of mid-ribs. The incipient axis will not, in 

 this case, be inclosed by the rolled-up fronds; but will con- 

 tinue exposed. Survival of the fittest will favour the genesis 

 of a type, in which those portions of the successive mid-ribs 

 that enter into the continuous bond, become more bulky than 

 the disengaged portions of the mid-ribs: the individuals 

 which thrive and have the best chances of leaving offspring, 

 being, by the hypothesis, individuals having axes stiff 

 enough to raise their foliage above that of their fellows. 

 At the same time, under the same influences, there will tend 

 to result an elongation of those portions of the mid-ribs, 

 which become parts of the incipient axis; seeing that it will 

 profit the plant to have its leaves so far removed from one 

 another, as to prevent mutual interferences. Hence, from the 

 recumbent type there will evolve, by indirect equilibration 

 (§ 167), such modifications as are shown in Figs. 92, 93, 94; 

 the first of which is a slight advance on the ideal type 

 represented in Fig. 76, arising in the way described ; and 

 the others of which are actual plants — Haplomitrium Hookeri, 

 and Plagiochila decipiens. Thus the higher Archegoniates 

 show us how, along with an assumption of the upright atti- 

 tude, there does go on, as we see there must go on, a separa- 



