II 



MUSCINEJEHEPA TICJE MARC HAN TI ALES 



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 



F. 



FIG. 2. Riccia glauca. Development of the archegonium, X52S. A, Vertical section 

 through the growing point; x, apical cell; or, young archegonium; //, 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 little depressions or pits to be formed between the adja- 

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

 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. 



