SELA GIXELLA CE^ 



39 



Order i. — Selaginelle-^. 



In the genus Selaginella, the sole representative of the order, the 

 prothaUium appears to be already completely formed by the time the 

 megaspore is mature, but occupies only the apical portion of the cavity 

 of the spore ; the basal 

 portion is siill filled by an 

 undifferentiated mucilagi- 

 nous protoplasm, which 

 subsequently develops into 

 a cellular tissue, the second- 

 ai-y prothalluim^ or, as it 

 is termed by some writers, 

 the endosperjH. In some 

 species, at least, this struc- 

 ture is separated by a dia- 

 phragm from the true pro- 

 thallium. The prothallium 

 always produces a number 

 of archegones, sometimes 

 as many as thirty, which 

 arise in centrifugal succes- 

 sion on the exposed portion 

 of the prothallium, the one 

 formed first being at the 

 apex. The archegone ori- 

 ginates by division of a 

 superficial cell in a direction 

 parallel to the surface ; the 

 outer of these two then 

 divides into four cells, and 

 each of these again breaks 

 up by tangential division 

 into two. These form the 

 7ieckoi\\i^ archegone, which 

 therefore consists of four 

 rows, each composed of 

 two cells. The lower of 

 the two original cells becomes the venter of the archegone, and puts out 

 a slender prolongation between the neck-cells, which is separated as the 

 neck-cafial-ceil. Another very small portion is subsequently separated 



Fig. 20. — A — F, stages in the division of the microspore of 

 Selaginella caulescens Spr. v, sterile cell ; G, antherozoid 

 ( X 1400) ; H, vertical section of megaspore of S. Mar- 

 tensii Spr. ;/, prothallium with three archegones ; end, en- 

 dosperm ; e, exospore(x 165). (After Pfeffer.) 



