§ 186. Having the surface of the soil for a support and the air for a medium, terrestrial plants are mechanically circumstanced in a manner widely different from that in which aquatic plants are circumstanced. Instead of being buoyed up by a surrounding fluid of specific gravity equal to their own, they have to erect themselves into a rare fluid which yields no appreciable support. Further, they are dissimilarly conditioned in having two sources of nutriment in place of one. Unlike the _Algæ_, which derive all the materials for their tissues from the water bathing their entire surfaces, and use their roots only for attachment, most of the plants which cover the Earth’s surface, absorb part of their food through their imbedded roots and part through their exposed leaves. These two marked unlikenesses in the relations to surrounding conditions, profoundly affect the respective modes of growth. We must duly bear them in mind while studying the further advance of composition. [Illustration: Figs. 41–44.] [Illustration: Figs. 45–49.] The class of plants to which we now turn--that of the _Archegoniatæ_--is nearly related by its lower members to the classes above dealt with: so much so, that some of the inferior liverworts are quite licheniform, and are often mistaken for lichens. Passing over these, let us recommence our synthesis with such members of the class as repeat those indications of progress towards a higher composition, which we have just observed among the more-developed _Algæ_. The _Jungermanniaceæ_ furnish us with a series of types, clearly indicating the transition from an aggregate of the second order to an aggregate of the third order. Figs. 41 and 42, indicate the structure among the lowest of this group. Here there is but an incomplete development of the second order of aggregate. The frond grows as irregularly as the thallus of a lichen: it is indefinite 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 proliferous growth; and occasionally, as shown in Fig. 46, representing an enlarged portion, the growth is doubly-proliferous.