70 



ELEMENTARY STUDIES IN BOTANY 



epidermis very resistant to the passage of water from the 

 interior of the plant to the surface. In other words, the 

 epidermis is in effect a waterproof layer, not to prevent 

 water from entering the plant, but to prevent water from 

 leaving the plant. 



A compact, prostrate (dorsiventral) body, ensheathed by 

 an epidermis, is well-equipped for exposure to the air, es- 



pecially if the air is not 

 very dry. 



The dorsiventral posi- 

 tion develops differences 

 in different regions of the 

 body. The dorsal side of 

 the body is exposed to 

 light, and as the light 

 reaches the cells, chloro- 

 phyll is formed, and such 

 cells are equipped for 

 photosynthesis (Fig. 52). 



^ J 



If the body is Very thin, 

 ,. , . . 



cups containing gemma; ; the rhizoids arising light may penetrate it 

 from the ventral surface are also shown ; B, nnrrmlptplv anH all thp 

 a section through a part of one of the disks, j61 ^' 



showing the short-stalked antheridia in flask- cells will be green. But 

 shaped 'cavities that open on the upper sur- 

 face of the disk ; C, the outline of a gemma. Ordinarily the body IS SO 



thick that light of suffi- 



cient intensity does not penetrate through it, so that a 

 certain number of layers of cells on the ventral side 

 may not contain chlorophyll (Fig. 52). In this way the 

 body is differentiated into two regions : a dorsal region 

 of green cells doing the work of carbohydrate manufac- 

 ture; and a ventral region of colorless cells. Another 

 form of differentiation is seen in the structures produced 

 by the two surfaces. The dorsal surface, with its free 

 exposure to the air, develops the sex-organs and spores, 

 for the spores are dispersed by currents of air (Figs. 53 and 



FIG. 53. Marchantia: A, a plant bearing on 

 its dorsal surface three long-stalked disks 

 containing antheridia, and also two small 



