1893.] 



Morphology of Spore-producing Members. 



21 



from the sterile tissue which forms the partition of the synangium, 

 or from the wall, while the cells which form the partition are similar 

 in their origin to the sporogenous cells. From the arrangement of the 

 cells of these sporogenous masses it seems not improbable that each 

 mass may be referable in origin to a single cell, but this has not been 

 proved to be constantly the case. All the cells of the sporogenous 

 tissue do not arrive at maturity, but here, as in Equisetum, a con- 

 >iderable number, serving as a diffused tapetum, become disorganised 

 Avithout forming spores. There is no clearly-defined tapetum in 

 Tmesipteris. The leaf lobes begin to be formed almost simultaneously 

 with the synangium, and appear as lateral growths immediately 

 below the apex of the sporangiophore ; their further development 

 presents no characters of special note. 



The synangium of Psilotum originates in essentially a similar 

 manner, being formed from the upper surface of the sporangiophore, 

 immediately below its apex. The details are different in accordance 

 with the trilocular character of the synangium : but, as regards the 

 structure of the wall and septa, the absence of any strict external 

 limit of the sporogenous masses, and of any definite tapetum, as well 

 as in the fact that a "considerable proportion of cells of the sporo- 

 genous masses become disorganised without forming spores, Psilotum 

 corresponds to Tmesipteris. Each sporogenous mass appears to be 

 referable in origin to a single archesporial cell. 



On the ground of the observations of internal development, of 

 which the above are the essential features, I agree with the conclu- 

 sion of Solms that the whole sporangiophore is of foliar nature, and 

 that the synangium is a growth from its upper surface. The presence 

 of the two lateral leaf-lobes need be no obstacle to this conclusion, 

 while they serve an obvious purpose in protecting the synangium 

 when young, more completely than a leaf of the form of the sterile 

 leaves of these plants could possibly do. 



For purposes of comparison with allied forms, Tmesipteris should 

 be taken first : and the correspondence is most close with Lepido- 

 | dendron, a fact which has a special interest since this genus and the 

 Psilotaceaa are both very ancient types. In Lepidodendron the 

 sporangium is very large ; it is narrow and elongated in a radial 

 direction, extending a considerable distance along the upper surface 

 > of the leaf. I have already communicated to the Society the fact 

 1 that trabecular extend in Lepidodendron from the base of the sporan- 

 gium far up into the mass of spores (loc. cit., p. 272), and have com- 

 pared these with the trabecules in the sporangium of Isoetes. Neither 

 of these sporangia are, however, completely partitioned. I now 

 suggest that comparatively slight modification of the condition in 

 Lepidodendron would produce the state of things seen in Tmesipteris : 

 if the sterile trabecular of Lepidodendron were consolidated into a 



