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 



