THE MORPHOLOGICAL COMPOSITION OF PLANTS. 55 



of proliferously-growing fronds, corresponds with the struc- 

 ture 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 unable to maintain an erect posi- 

 tion. Hence there will result the recumbent, continuously- 

 rooted stem, which these types exhibit : an embryo phaenogam 

 having the weakness of an embryo.* 



A natural concomitant of the mode of growth here de- 

 scribed, is that the stem, while it increases longitudinally, 

 increases scarcely at all transversely: hence the old name 

 Acrogens. Clearly the transverse development of a stem is 

 the correlative, partly of its function as a channel of circula- 

 tion, and partly of its function as a mechanical support. 

 That an axis may lift its attached leaves into the air, implies 

 thickness and solidity proportionate to the mass of such 

 leaves; and an increase of its sap-vessels, also proportionate 

 to the mass of such leaves, is necessitated when the roots are 

 all at one end and the leaves at the other. But in the 

 generality of Acrogens, these conditions, under which arises 

 the necessity for transverse growth of the axis, are absent 

 wholly or in great part. The stem habitually creeps below 

 the surface, or lies prone upon the surface; and where it 

 grows in a vertical or inclined direction, does this by attach- 

 ing itself to a vertical or inclined object. Moreover, throwing 

 out rootlets, as it mostly does, at intervals throughout its 

 length, it is not called upon in any considerable degree, to 

 transfer nutritive materials from one of its ends to the other. 



* To this interpretation it is objected that "the more developed Junger- 

 manniacece" do not appear to have arisen from the lower forms of Junger- 

 manniaccce — that is to say, from such lower forms as are now existing. It 

 may, however, be contended that this fact does not exclude the interpretation 

 given ; since the higher forms may well have been evolved, not f om any of 

 the lower forms we now know, but from lower forms which have become 

 extinct. This, indeed, is the implication of the evolutionary process as 

 pointed out in the note to Chap. I. If then we assume some early type of 

 intermediate structure, the explanation may not improbably hold. 



