58 MORPHOLOGICAL DEVELOPMENT. 



an axis ai-ises, when the axis comes to be a channel of circu- 

 lation between all the roots at one of its extremities and all 

 the leaves at the other ; and if this lateral bulging must in- 

 crease, as fast as the quantity of foliage to be brought in 

 communication with the roots increases — especially if such 

 foliage has at the same time to be raised high above the 

 earth's surface ; what must happen to a plant constructed in 

 the manner just described ? The elder fronds or foliar or- 

 gans, ensheathing those within them, as well as the incipient 

 axis serving as a bond of union, are at first of such circum- 

 ference only as suffices to inclose these undeveloped parts. 

 What, then, wOl take place when the inclosed parts grow — • 

 when the axis thickens while it elongates ? Evidently the 

 earliest-formed sheaths, not being large enough for the 

 sweUing axis, must burst ; and evidently each of the later- 

 formed sheaths must, in its turn, do the like. There must 

 result a gradual exfoliation of the successive sheaths, lilio 

 that indicated as beginning in the above figure of Dendro- 

 hium ; 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 

 helps 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 

 stifihess ; then, the only way in which the requisite stiifiiess 

 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 roUed-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 



