850 MR JOHN M'LEAN THOMPSON ON 



Development of the Sorus. 



Division begins in the original wedge-shaped, marginal initial-cell, and a group of 

 wedge-shaped cells is formed (figs. 21-23). One cell in the centre of this group takes 

 the lead and a new wedge-shaped, marginal initial-cell arises (fig. 24). Somewhere 

 back from this new initial a superficial cell becomes enlarged and distinctly protruded, 

 and marks the point of origin of the lower, abaxial lip of the indusium. After a 

 few more segmentations — mainly from the upper surface of the new marginal cell — 

 have taken place, the upper, adaxial lip of the indusium appears as a large projecting 

 cell (fig. 25). The indusium having thus been established, the remainder of the 

 group of marginal cells is destined to give rise to the soral-receptacle. But, as will 

 be seen from fig. 26, the origin of the upper part of the indusium may be delayed, 

 although the lower part is well advanced. It will be observed that the marginal 

 initial-cell is still recognisable. A slightly different condition is represented in 

 fig. 27. The upper adaxial lip of the indusium has just appeared, but is removed 

 some distance from the margin. More space is thus devoted to the receptacle. A 

 single initial-cell can no longer be recognised, but a sporangium initial has arisen 

 near the centre of the receptacle. 



A median section cut in a plane at right angles to the plane of the preceding 

 sections is represented in fig. 28. A sporangium initial occupies a median position 

 on the receptacle, and the indusium is seen to be continuous on the lower surface and 

 sides of the receptacle. When figs. 27 and 28 are compared, it is evident that the 

 indusium is continuous around the receptacle, but that the upper lip may lag back at 

 an early stage of development and be withdrawn to the upper surface of the lamina. 



From such sections as these it would appear that although it cannot be said 

 that the marginal initial-cell becomes the initial of the first-formed sporangium, 

 the first sporangium initial is, however, almost or quite median on the receptacle. 



But the condition represented in fig. 29 is sometimes found, and when the sections 

 to right and left in the series are counted it is found that such a section as this is 

 truly the median section of the sorus. It is impossible to say which is the first 

 sporangium initial, but the group of initials is, at least, truly marginal, though the 

 upper lip of the indusium is clearly withdrawn on to the upper surface of the lamina. 



A median section of a much more advanced sorus is depicted in fig. 30. The 

 indusium is now well advanced and the receptacle is bulky and expansive. It is 

 evident that the sorus is a " mixed" one, but that the middle sporangium is slightly 

 more advanced than any of the other sporangia. There is, in fact, a slightly 

 basipetal succession. 



All the marginal sori of this age which were examined were of this "mixed" 

 character, and showed some trace of the basipetal succession. 



A median section of an almost full-grown sorus is represented in fig. 34. The 

 "mixed" character of the sorus is now very prominent, but a glance over the 



