THE MORPHOLOGICAL COMPOSITION OF PLANTS. 33 



transition from an aggregate of the second order to an aggre- 

 gate of the third order. Figs. 41, and 42, indicate the struc- 

 ture among the lowest of this group. Here there is but an in- 

 complete development of the second order of aggregate. The 



frond grows as irregularly as the thallus of a lichen : it is in- 

 definite in size and outline, spreading hither or thither as 

 the conditions favour. Moreover, it lacks the differentiations 

 required to subordinate its parts to the whole : it is uniformly 

 cellular, having neither mid-rib nor veins; and it puts out 

 rootlets indifferently from all parts of its under-surface. In 

 Fig. 43, Pellia epiphylla, we have an advance on this type. 

 There is here, as shown in the transverse section, Fig. 44, a 

 thickening of the frond along its central portion, producing 

 something like an approach towards a mid-rib; and from 

 this the rootlets are chiefly given off. The outline, too, is 

 much less irregular; whence results greater distinctness of 

 the individuality. A further step is displayed in Metzgeria 

 furcata, Fig. 45. The frond of this plant, comparatively well 

 integrated by the distribution of its substance around a 

 decided mid-rib, and by its comparatively-definite outlines, 

 produces secondary fronds. There is what is called prolifer- 

 ous growth ; and occasionally, as shown in Fig. 46, represent- 

 ing an enlarged portion, the growth is doubly-proliferous. In 

 these cases, however, the tertiary aggregate, so far as it is 

 formed, is but very feebly integrated; and its integration is 

 but temporary. For not only do these younger fronds that 

 bud out from the mid-ribs of older fronds, develop rootlets of 

 their own ; but as soon as they are well grown and adequately 

 rooted, they dissolve their connexions with the parent-fronds, 

 49 



