THE MORPHOLOGICAL COMPOSITION OF PLANTS. 51 



that the supply of mineral matters obtained by the rootleig 

 suffices for the full development of the secondary frond, there 

 is a likelihood that the growth from it of a tertiary frond, will 

 become an habitual characteristic of the variety. Along with 

 the establishment of such a tertiary frond, as sho\\Ti in Fig. 

 74, there must arise a further development of mid- rib in the 

 primary frond, as well as in the secondary frond— a develop- 

 ment which must bring with it a greater integration of the 

 two ; while, simultaneously, extra growth will take j^lace in 

 such of the rootlets as are most directly connected with this 

 main channel of circulation. Without further explanation it 

 will be seen, on inspecting Figs. 75 and 76, that there may 

 in this manner result an integrated scries of fronds, placed 

 alternately on opposite sides of a connecting vascular struc- 

 ture. That this connecting vascular structure will, as shown 

 in the figures, become more distinct from the foliar surfaces as 

 these multiply, is no unwarranted assumption ; for we have 

 seen in compound-leaved plants, how, under analogous con- 

 ditions, mid-ribs become developed into separate supporting 

 parts, which acquire some of the characters of axes while as- 

 suming their functions. And now mark how clearly 

 the structure thus built up by integration of proliferously- 

 growing fronds, corresponds with the structure of the more- 

 developed Jungermanniacece. Each of the fronds successively 

 produced, repeating the characters of its parent, will bear 

 roots ; and will bear them in homologous places, as shown. 

 Further, the united mid-ribs having but very little rigidity, 

 will be imable to maintain an erect position. Hence there 

 will result the recumbent, continuously-rooted stem, whicu 

 these t}^es exhibit. Xay, the parallelism is more complete 

 than the figures show. To avoid confusion, the fronds thus 

 supposed to be progressively integrated, have been repre- 

 eented as simple. But, as shown in Fig. 45, these lower 

 types ordinarily have fronds which divide dichotomously, in 

 such way that one division is larger than the other ; and thi« 



