DEVELOPMENT OF PLANTS 



273 



tion of a cuticle. In most species the air spaces arising among the 

 upper cells of the thallus increase greatly in size and the epidermal 

 cells by vertical divisions keep pace with this enlargement and 

 thus arch over the air cavities, forming, however, a small opening 

 in the epidermal layer that permits a free circulation of the air 

 (Fig. 1 80, B}. These openings are suggestive of stomata and 

 function in the same way, though they have originated in an 

 entirely different manner. The lower cells of the thallus are 

 compact and doubtless serve as storage cells, being nearly or 

 quite destitute of chlorophyll. The stimulus of the soil causes 



FIG. 180. Structure of the thallus of Ricciocarpus: A, section of the 

 thallus, showing the apical cell, x, forming cells that by further division de- 

 velop into plates of cells separated by air spaces, j. At the left the plates 

 thus formed are seen curving over the apical cell. B, an older portion of 

 the upper part of the thallus. The air spaces, j, are greatly enlarged and 

 the upper cells of the vertical plates have divided, arching over the air spaces 

 but leaving small openings which permit the entrance of air for photosyn- 

 thesis. I. D. Cardiff. 



the lower epidermal cells of the thallus to form numerous smooth 

 or pitted rhizoids that are suggestive of the root-hairs of the higher 

 plants, anchoring the plant to the substratum and assisting in the 

 absorption of the earth substances. Plates of cells similar in 

 origin to those occurring on the upper surface are also found 

 on the under side of the thallus (Fig. 180, A). These plates 

 curve up around the growing point and doubtless protect it 

 against drought, in which work they are assisted by the mucilage 



