ITS INCONSTANT FEATURES 



105 



constant, notwithstanding that it x shows some constancy of character in 

 certain circles of affinity. Sometimes it is not differentiated at all, a condition 

 which holds throughout the Bryophyta : in other cases it may appear as 

 a more or less definite band of cells, which originates from the tissues 

 surrounding the sporogenous cell or group of cells, sometimes from the 

 sporogenous group itself. In some cases a large number of cells of the 

 sporogenous groups act like a diffused tapetum, becoming disintegrated 

 during the development of the spores (Ophioglossum, Psilotum) : in Equisetum 

 both a diffused tapetum of 



this nature is found, and also B 



a definite single-layered tape- 

 tum, which originates outside 

 the sporogenous group. In 

 Lycopodivm, and in the Mar- 

 attiaceae the tapetum arises 

 from cells outside the sporo- 

 genous group : in other cases, 

 such as Selaginella and the 

 Leptosporangiate Ferns, the 

 tapetum may consist of cells 

 cut off from the sporogenous 

 cell or cell-group. There is 

 indeed good reason to think 

 that there has been a pro- 

 gressive change of origin of 

 the tapetum within certain 

 circles of affinity : speaking 

 generally it may be said that 

 indefinite and non-specialised 

 nutritive arrangements are 

 characteristic of larger and 

 probably primitive sporangia, 

 but more definite tapetal 

 layers are found in the smaller 

 and probably derivative : and 

 further, that while in sporangia 

 of relatively large size the 



tapetum usually originates outside the sporogenous group, in smaller 

 sporangia of the same affinity it may be cut off from the sporogenous cell 

 or cell-group. Such a change appears to be illustrated by comparison of 

 Lycopodium with Selaginella, and of the Marattiaceae with the Lepto- 

 sporangiate Ferns. The result of such comparisons is that the tapetum, 

 however uniform it may be in function, is found to be variable both in 

 occurrence and in origin, and accordingly it cannot be regarded as an 

 essential or constant feature of the sporangium. 



Loxsoma Cunninghami, Br. E, young sorus. A , rather 

 older. &, vertical section of the base of the receptacle, showing 

 young sporangia. C, D, mature sporangia, showing the 

 1 annulus, and distal point of dehiscence 

 5x250. C+Dxso. 



