24 MORPHOLOGICAL DEVELOPMENT. 



the Sargassum, or common gulf- weed, this tertiary degree oi 

 composition is far more completely displayed, so as to pro- 

 duce among Thallogens a type of structure closely simulating 

 that of the higher j)lants, let us now pass to the considera- 

 tion of these higher plants. 



§ 186. Having the surface of the soil for a support and tho 

 air for a medium, terrestrial plants are mechanically circum' 

 stanced in a manner widely different from that in which 

 aquatic plants are circumstanced. Instead of being buoyed 

 up by a surrounding fluid of specific gra\dty equal to their 

 own, the}^ have to erect themselves into a rare fluid which 

 yields no appreciable support. Further, they are dis- 

 similarly conditioned in having two sources of nutriment in 

 place of one. Unlike the Algct, which derive all the mate- 

 rials 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 rela- 

 tions to surrounding conditions, profoundly affect the respec- 

 tive modes of growth. "We must duly bear them in mind 

 while studying the further advance of composition. 



The class of plants to which we now turn — that of Acrogens 

 — 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 analysis 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 Alffce. The JungermanniacccB 

 furnish us with a series of types, clearly indicating the transi- 

 tion 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 incom- 

 plete development of the second order of aggregate. The 



