312 LYCOPODIALES 
but clearly upon its upper surface as a transversely extended cushion 
(Fig. 157 a). In median radial section it appears as a convex growth, in 
which a central row of three cells, the result of periclinal division of one 
parent cell, is dominant (Fig. 156): of these the middle cell is of arche- 
sporial character. A tangential section of a sporangium of similar age 
(Fig. 157 8) shows that there are a number of these archesporial cells: 
in the example shown there are seven: but the number is not constant, 
as is shown by comparison of various tangential sections, and supported 
by sections cut transversely (Fig. 157 c). The young sporangium consists 
thus of a single tangential row of archesporial célls, covered in completely 
Fic. 157. 
Lycopodium Selago. A=young sporangium seen in superficial view: st=stem; 
2=sporophyll. =tangengial section of a similar sporangium, the cells numbered i, 
ii, iil correspond to those similarly marked in Fig. 156. C=a sporangium of like age in 
transverse section, as along a line s, s, in Fig. B. The archesporial cells are shaded. 
D=an older sporangium, in radial section, showing the spore-mother-cells separated, 
before tetrad-division. A,B, Cx200. Dx 100. 
by a single layer of cells forming the sporangial wall, and supported 
below by cells which grow more actively in the middle region of the 
sporangium, thus leading to the curved form which it assumes later. It 
is clear also that all the essential parts of the sporangium originate from 
several superficial cells of the sporophyll, and that it is impossible to refer 
them in origin to any single parent cell. 
At first the parts thus laid down often grow uniformly, so that their 
mode of origin may still be traced in a more advanced state: but later the 
more numerous divisions are less regular. Superficially they result in the 
formation of a sporangial wall, composed of three layers, or of more 
towards the base of the sporangium (Fig. 157 D): of these the innermost is 
the transitory tapetum: the tapetal investment of the sporogenous tissue is 
completed by development of the adjacent cells of the sub-archesporial 
‘ial 
