DEVELOPMENT OF PLANTS 



259 



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. 180, B). These open- 

 ings 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 stor- 



FiG. 180. Structure of the thallus of Ricciocarpiis: 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. Cardifl. 



age cells, being nearly or quite destitute of chlorophyll. The 

 stimulus of the soil causes 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 sub- 

 stratum 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 



