ITS INCONSTANT FEATURES 105 
constant, notwithstanding that it 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 (Op/ioglossum, Psilotum): in Egquisetum 
both a diffused tapetum of 
this nature is found, and also 
a definite single-layered tape- 
tum, which originates outside 
the sporogenous group. In 
Lycopodium, and in the Mar- 
attiaceae the tapetum arises 
from cells outside the sporo- 
genous group: in other cases, 
such as Se/aginella 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 
4 B 
probably primitive sporangia, c D E 
but more definite  tapetal Fic. 60. 
layers are found in the smaller Loxsoma Cunninghami, Br. &, young sorus. 4, rather 
: : older. 4, vertical section of the base of the receptacle, showing 
and probably derivative: and young sporangia. C, , mature sporangia, showing the 
10: : incompletely indurated annulus, and distal point of dehiscence 
further, that while in sporangla (x). A+tExabout 20. Bx250. C+D 50. 
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. 
