THE MORPHOLOGICAL COMPOSITION OF TLA NTS. 



25 



frond grows as irregularly as tlie tliallus of a. lichen : it is in- 

 definite in size and outline, spreading hither or thither as the 

 conditions favour. Moreover, it lacks the differentiations re- 



quirt^d 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, Jungermannia 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, pro- 

 ducing 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 Junger- 

 mannia f areata, Fig. 45. The frond of tliis plant, compara- 

 tively 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 

 proliferous growth ; and, occasionally, as shown in Fig. 46, 

 representing an enlarged portion, the growth is doubly-pro- 

 liferous. In these cases, however, the tertiary aggregate, so 

 far as it is formed, is but very feebly integrated ; and its in- 

 tegration 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, and become quite independent. From 



these transitional forms we pass, in the higher Jangerman- 

 niacecp, to forms composed of man}^ fronds that are perman- 

 ently united by a continuous stem. A more-developed ag- 



