208 



BOTANY 



grow, and the overlapping lamellae, which are formed by the rapid growth of 

 the free margin of the segments, and curve upward over the apex. These 

 lamellae are very inconspicuous in certain species (e.g., It. glauca), while in 

 others they subsequently split in the middle, and form two rows of scales like 

 those of the Marchantiacese. 



The dorsal segments grow much more rapidly, and divisions occur in all direc- 

 tions, so that the thallus becomes thick, and the upper cells are arranged in more 

 or less definite vertical rows, which separate at an early period, and give rise to 

 narrow air-spaces between the rows of green cells. In some species, these air- 

 spaces become much larger, and approach the condition found in the Marchan- 

 tiaceae. The green cells are thus brought into direct contact with the air 

 containing the CO% necessary for photosynthesis. The terminal cell of each 

 row is usually colorless, and somewhat enlarged, so that a sort of epidermis is 

 developed. Differences in the turgescence of these cells probably regulate, to 

 some extent, the communication between the air-spaces and the atmosphere 

 outside. In Ricciocarpus, where the air-spaces are large, there is a definite 

 epidermis with pores like those of the higher Marchantiaceae. 



c 



D. 



FIG. 166. Fimbriaria sp. A, part of a vertical section of a youug antheridial 

 receptacle, showing two very young antheridia 6. B-E, older stages of the 

 antheridium (X 420). 



Sex-organs. The sexual organs of Riccia (Fig. 165) are borne upon the 

 dorsal surface of the thallus, but, owing to the growth of the tissue about them, 

 they are surrounded by an envelope, which, in the case of the antheridium, 

 extends above its apex in the form of a tube. Both antheridium and arche- 

 gonium arise from similar superficial cells, and closely resemble each other at 

 first. The development of the archegonium conforms to the regular type. At 

 maturity it has an enlarged venter and elongated neck. 



The antheridium, after a short basal cell is cut off, divides by a series 

 of transverse divisions, which are followed in each of the segments by two 

 intersecting vertical walls : a periclinal wall in each of the segment -quadrants 

 separates a central cell from a peripheral one. The central cell, thus produced 

 by further divisions, gives rise to the sperm-cells, which are very numerous. 



The Embryo. The globular egg divides first by a transverse wall, and then 

 undergoes quadrant and octant divisions by walls passing through its centre 

 (Fig. 171). After several other divisions, a single layer of peripheral cells is 

 cut off, and all the central mass of cells becomes the archesporium, all of whose 

 cells develop spores. The outer sterile cells become more or less completely 



