Nutrition of Multicellular Plants - 239 



transmits substances up and down between 

 the leaves and roots. 



NUTRITION OF THE BRYOPHYTES 



The simplest and most primitive of terres- 

 trial plants are the Bryophyta (p. 610). This 

 phylum consists of two groups: (1) the liver- 

 worts (class Hepaticae, p. 611); and (2) the 

 mosses (class Musci, Fig. 12-11). None of the 

 Bryophyta is very well adapted to land con- 

 ditions — the liverworts even less so than the 

 mosses. In all the Bryophyta the main burden 

 of nutrition falls upon the gametophyte gen- 

 eration — the sporophytes being relatively 

 small and virtually dependent. Conse- 

 quently the nutritive processes of only the 

 gametophytes will be discussed. 



The Liverworts. The liverworts are small 

 semiterrestrial plants, unfamiliar to most 

 people. The flat thallus body (Fig. 13-4), 

 which seldom has an area of more than one 

 or two square inches, grows in contact with 

 the moist ground. Only the upper surface of 



the thallus is exposed to light, and the lower 

 surface, which lies in contact with the 

 ground, sends numerous colorless rhizoids 

 downward into the soil. Usually many plants 

 lie crowded closely together, completely cov- 

 ering the moist ground in a region that is 

 frequently flooded by a neighboring spring, 

 or by seepage from a hillside. 



These primitive land plants manage to 

 survive even in the absence of true roots, 

 stems, and leaves. The thallus displays a 

 fairly complex internal structure, with con- 

 siderable specialization among the cells (Fig. 

 13-5). To protect the thallus from the strong 

 light and heat of the sun, and to prevent the 

 plant from losing more water than it can 

 absorb through the underlying rhizoids, the 

 thallus is covered by a layer of epidermoid 

 tissue, which is especially well developed on 

 the upper surface. The outer walls of the 

 epidermoid cells are thickened and cutinized 

 (waxy), to prevent an indiscriminate evapo- 

 ration of water from the delicate internal 

 tissues as the sun beats down upon the plant. 



Fig. 13-4. A group of liverworts (Marchantia) growing near a spring. Note 

 how these primitive land plants crowd together, mutually protecting themselves 

 from desiccation during dry weather. In the cuplike bodies (gemma cups), which 

 can be seen on the upper surfaces of some of the thalli, budlike reproductive 

 bodies (gemmae) are produced and liberated periodically. The background of 

 this photograph was retouched to give more contrast. 



