32 MORPHOLOGICAL DEVELOPMENT. 



order, well individuated by their forms and structures, are 

 united together; and the plant composed of them is thus 

 rendered, in so far, an aggregate of the third order. 



Just noting that in certain of the most-developed Algce, as 

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

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

 duce among Thallophytes a type of structure closely simulat- 

 ing that of the higher plants, let us now pass to the considera- 

 tion of these higher plants. 



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

 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 gravity equal to their 

 own, they 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 Algce, 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 the 

 Archegoniatce — 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 ob- 

 served among the more-developed Algce. The Jungerman- 

 niacecB furnish us with a series of types, clearly indicating the 



