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 B) 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 cells, covered in completely 



Yr ~*S2 



. 



FIG. 157. 



Lycopodium Selago. A= young sporangium seen in superficial view: s< = stem; 

 /=sporophyll. .Z> = tangential section of a similar sporangium, the cells numbered i, 

 ii, iii 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. 

 Z> = an older sporangium, in radial section, showing the spore-mother-cells separated, 

 before tetrad-division. A , B, C X 200. D X 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 



