THE SPORANGIUM 641 
this hoop became simplified to a single cell-row; and that it changed its” 
position in accordance with the soral arrangements, being oblique in some 
Simplices and in all Gradatae, first with median but subsequently with lateral 
dehiscence ; and that finally it swung again into the vertical position in the 
Mixtae, but with the stomium still remaining lateral. 
Passing to the contents of the sporangium, these are derived from the 
archesporium, which in all Ferns, with occasional excéptions in abnormal 
Eusporangiates, consists of a single cell. From this in the Leptosporan- 
giatae the tapetum is first cut off peripherally, and the central cell which 
remains divides up into cells which are commonly found to number some 
power of two. This is a consequence of the successive divisions occurring 
‘as a rule simultaneously within the sporogenous group, and the result is 
that the spore-mother-cells number 2, 4, 8, 16, 32, 64, etc. Since each of 
these undergoes tetrad divisions, the numbers of spores produced may as 
a rule be 4, 8, 16, 32, 64, 128, 256, 512, etc. These may be styled the 
typical numbers. 
There are two ways of computing the output of spores from a single 
sporangium: either by examining preparations of sporangia with spore- 
mother-cells, and estimating their number, or by actual counting of the 
spores produced. Where the output of spores is small a reasonable 
degree of precision is to be expected by either method; but where the 
numbers are large there are various sources of error, and the results must 
be taken as mere approximations. The results of such computations will 
now be given for various Ferns: the estimates for the Marattiaceae have 
been made from study of sections traversing the sporogenous group before 
tetrad-division. but in all the rest the results are those from direct 
counting of the mature spores (see Table, p. 642). 
In estimating the value of these results it is first to be noted that the 
largest numbers are associated with complete synangial fusion, a smaller 
number being found in Azgéopteris where the sporangia are separate than 
in any of the synangial genera of living Marattiaceae. The potential 
number estimated for Gleichenta flabellata is nearly equivalent to that of 
Angiopteris, though the actual countings run considerably below ; still they 
care in excess of those for any other Leptosporangiate Fern observed, and 
as this is seen in a Fern in which the type of sorus is the same as in 
Angiopteris, it strengthens the affinity between these two genera, notwith- 
standing that the one is, as regards the mode of segmentation of the 
young sporangium, a typical Eusporangiate, while the other shows 
essentially the segmentation of the Leptosporangiates. 
From Géleichenia, as we pass through the table, successively lower 
numbers are seen, and it is obvious that the larger numbers are 
associated with those Ferns which on other grounds are held to be 
relatively primitive. Of the Simplices examined none have a lower typical 
number than 128: and in this connection it may be noted that a rough 
estimate of the spore-output from a sporangium of Zygopéeris, after 
; 2S 
