II 



MUSCINEJE—HEPATICM—MARCHANTIALES 



25 



that the margin is occupied by a group of cells that look very 

 much alike. Sometimes one of these cells is somewhat larger 

 than the others, but more commonly it is impossible to decide 

 with certainty that a single initial is present. From a com- 

 parison of the two sections it is at once evident that the initial 

 cells have nearly the form of the segment of a disc, and that in 

 addition to the dorsal and ventral segments lateral ones are cut 

 off as well. In the region just back of the apex the tissue of 



Fig. 2. — Riccia glauca. Development of the archegonium, XS25. A, Vertical section 

 through the growing point; x, apical cell; ar, young archegonium; W, ventral 

 lamellae; B-F, successive stages in the development of the archegonium, seen in 

 longitudinal section; G, cross-section of young archegonium (diagrammatic). 



the thallus is compact, but in the older parts a modification is 

 observable both on the dorsal and ventral surfaces. In the 

 former, a short distance from the growing point, the superficial 

 cells project in a papillate manner above the surface. This 

 causes httle' depressions or pits to be formed between the adja- 

 cent cells (Fig. 3, C). The subsequent divisions in the papillse 

 are all transverse, and this transforms each papillate surface cell 

 into a row of cells which, as it elongates, causes the pits 

 between it and the adjacent ones to become deep but narrow 

 air-channels, so that in the older parts of the thallus the upper 

 portion is composed of closely-set vertical rows of chlorophyll- 

 bearing cells separated by narrow clefts opening at the surface. 



