SPORE-PRODUCING MEMBERS 



317 



define the future sporogenous tksue from the sporangial wall, but that by 



further periclinal divisions of the superficial cells additions may be made 



to it. If this be so, then 



the distinction between A 



the two sources of origin 



of the tapetum does not 



appear so marked as at 



first sight it might appear 



to be. 



In the microsporangia 

 all the cells of the sporo- 

 genous group may under- 

 go the tetrad-division, 

 and form microspores ; 

 but Miss Lyon found 

 that in S. apus not more 

 than five-sixths of them 

 were fertile, the rest dis- 

 appear. In the mega- 

 sporangia, as a rule, a 

 single cell is early differ- 

 entiated by its denser 

 protoplasm from the rest : 



FIG. 163. 



Selaginella spinnlosa. A, - = radial sections through young 

 sporangia of successive ages. C=a transverse section of one more 

 advanced. D a tangential section. E=a radial section of an older 

 sporangium showing all its essential parts, together with the ligule and 

 part of the sporophyll. A,,C, 0x350. .fix 200. 



this alone undergoes the 



tetrad-division, and forms megaspores (Fig. 164). But in S. rupestris a 

 smaller number of megaspores, or even only one, may come to maturity : 



in S. apus, however, two mother-cells 

 may become matured, and eight mega- 

 spores be thus formed in one sporangium. 

 These fluctuations have their interesting 

 bearing upon the origin of the hetero- 

 sporous differentiation, showing that there 

 is some margin of variation in the num- 

 ber of spore-mother-cells which are fertile 

 even in forms now living. 



The facts relating to the sporangium 

 in Selaginella^ though imperfectly known 

 for the genus at large, show that in 

 position and in general plan the spor- 

 angium is of the usual Lycopod type ; 

 but that its dimensions are smaller than 

 is usually the case in Lycopodium : the 

 difference in origin of the tapetum is probably related to the smaller size 

 of the whole sporangium. The heterosporous condition appears to have 

 brought with it only minor modifications of the original sporangial type. 



FIG. 164. 



Selaginella spinulosa, A. Br. Section of 

 megasporangium showing the single fertile 

 tetrad still very small, and the rest of the 

 sporogenous cells arrested. X 100. 



