484 MOSSES AND FERNS chap. 



large as well as the small specimens a single initial is usually 

 present, but its segmentation does not appear to be strictly 

 regular, and it is difficult to refer the whole meristem to the 

 activity of one parent cell. . . . When a leaf or sporangio- 

 phore is about to be formed, certain of the superficial cells 

 increase in size, and undergo both periclinal and anticlinal 

 divisions so as to form a massive outgrowth, the summit of 

 which is occupied, as seen in radial section, by a single larger 

 cell of a wedge -like or prismatic form. ... In these early 

 stages I find it impossible to say whether the part in question 

 will be a vegetative leaf or a sporangiophore, and even when 

 older it is still a matter of uncertainty. . . . Those which are to 

 develop as sporangiophores soon show an increase in thickness, 

 while they grow less in length ; an excrescence of the adaxial 

 surface soon becomes apparent (Fig. 252, A, sy), in which the 

 superficial cells are chiefly involved. . . . The superficial cells at 

 first form a rather regular series, which may be compared with the 

 cells which give rise to the sporangia in Lycopodium clavatum, 

 or in Isoetes : they undergo more or less regular divisions, 

 which, however, I have been unable to follow in detail : a band 

 of tissue some four or more layers in depth is thus produced. 

 About this period certain masses of cells assume the characters 

 of a sporogenous tissue : but though they can be recognised as 

 such by the character of the cells, it is extremely difficult to 

 define the actual limits of these sporogenous masses." 



In Tniesipteris there are normally two masses of sporo- 

 genous tissue corresponding to the two loculi in the mature 

 synangium ; in Psilotuni, which correspond closely with Tniesi- 

 pteris in other respects, there are three. Whether additions are 

 made to the sporogenous tissue from cells outside the original 

 archesporium was not determined with certainty, but Professor 

 Bower thinks it not improbable. In Psilotuni the young 

 archesporium is more clearly defined than in Tniesipteris, and it 

 seems not unlikely that each sporogenous mass is referable to 

 the division of a single primary archesporial cell. In both 

 genera some of the sporogenous cells do not develop spores, 

 but simply serve for the nourishment of the others, as in 

 Equisetuni. 



The fully-developed synangium has the outer walls of the 

 loculi composed of a single superficial layer of large cells, 

 beneath which are several layers of smaller ones (Fig. 252, D), 



