THE MORPHOLOGICAL COMPOSITION OF PLAXTS. 



59 



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 Jungermannia Hookeri, 

 and /. dedpiens. Thus the higher Acrogens show us how, 

 along with an assumption of the upright attitude, there does 

 go on, as we see there must go on, a separation of the leaf- 

 producing parts from the root-producing parts ; a greater 

 development of that connecting portion of the successive 

 fronds, by which they are kept in communication with the 

 roots, and raised above the ground ; and a consequent in- 

 creased differentiation of such connecting portion from the 

 parts attached to it. And this lateral bulging of the axis, 

 directly or indirectly consequent on its functions as a suppo.:! 



