246 



DEVELOPMENT OF RICCIOCARPUS 



epidermis. It is also interesting to note that the direct contact 

 of the cells with the atmosphere results for the first time in the 

 formation of a cuticle. In most species the air spaces arising 

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



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

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

 develop into plates of cells separated by air spaces, /. 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, /, 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 

 photosynthesis. I. D. Cardiff. 



the epidermal cells by vertical divisions keep pace with this en- 

 largement and thus arch over the air cavities, forming, however, 

 a small opening in the epidermal layer that permits a free circu- 

 lation of the air (Fig. 180, B). These openings are suggestive 

 of stomata and function in the same way, though they have origi- 

 nated 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 the lower epidermal cells of the thallus to form numer- 

 ous smooth or pitted rhizoids that are rather suggestive of the 

 root-hairs of the higher plants, anchoring the plant to the sub- 

 stratum and assisting in the absorption of the earth substances. 

 Plates of cells similar in origin to those occurring on the upper 



