610 ME. S. LE M. MOOKE'S STILDIES 



of rings similar to, but smaller than, those between the axial 

 cells themselves. 



Delessebia alata. — The midrib of the frond is composed of 

 several series of long wide cells with broad ends arranged in 

 longitudinal rows separated by smaller cells, of which some have 

 a wide, others a narrow lumen (fig. 35). Examination of the 

 larger cells by means of longitudinal and transverse sections of 

 the midrib of old parts of the frond, reveals a method of continuity 

 different from anything hitherto remarked upon. Reference 

 to figs. 35 and 3G will show that a large part of the floor and roof 

 of neighbour cells is closed up by membrane, round which at 

 certain points are placed small rings, to each of which runs a 

 stout cord of protoplasm. The rings are themselves covered in 

 with membrane similar to that which isolates the cells ; this 

 membrane is pierced either by several holes, in which case the 

 'strand of protoplasm is divided in passing it, or in some cases 

 where the ring is small, there is a single hole at the centre of the 

 membrane (figs. 37 and 38). All round the large membrane can 

 usually be traced with more or less ease a slender ring of highly 

 refractive substance, similar to that of which we have seen the 

 contour of the rings to be composed. Fig. 35 shows these matters 

 in transverse section ; in this case there are four small rings 

 visible, and apparently a fifth veiled behind a cloud of protoplasm ; 

 their number, however, varies between three and five. 



The only interpretation that can be put upon these facts is 

 that the large membrane surrounded by its fine ring of gleaming 

 protoplasm is homologous with the ring of Ptilota or Ceramium ; 

 and that the small rings are accessory structures. It is therefore 

 proposed to call the former the " primary " ring, and the small 

 rings " secondary " ones. The rings will be spoken of as " com- 

 pound " or " simple," according as they have or have not secondary 

 rings. 



In some cases it would perhaps seem that a simple ring is 

 interposed between two of the large cells. I am inclined to think 

 that fig. 40 represents such a ring, although it was not possible 

 to make out satisfactorily that there were not small secondary 

 rings at the circumference of the larger one. 



The primary ring is closely attached to the inner boundary of 

 the incomplete roof-and-floor wall of the cells, which is frequently 

 wavy in outline. Even when best developed, it is very slender ; 

 in profile view it can be easily recognized as a continuation of the 



