ALTERNATION OF GENERATIONS. 



131 



Fig. 63. (After Suminski.)— Spo- 

 rangium of Pteris serrulata. p, 

 pedicel ; c, capsule ; a, annulus ; 

 s, spore. 



speaking, consists tlierefore in an alternation of the sporophore 

 (asexual generation) with the oopliore (sexual generation) ; that 

 is, it consists of an alternation of 

 generations. An essentially similar 

 alternation of sporophore with oophore 

 occurs in all higher plants, though in 

 most cases it is so disguised as to es- 

 cape ordinary observation. 



The Sporangia and Spores. The 

 sporangia of Pteris (Figs. 63, 64) 

 arise upon a longitudinal thickening 

 of tissue situated on the under side of 

 the leaflets near their edges, and in- 

 cluding a marginal anastomosis of the 

 veins. This swelling is known as 

 the receptacle. Hairs are not uncom- 

 mon upon the under side of the leaf, 

 and some are found upon or near the 

 receptacle. On the latter arise structures, at first superficially 

 similar to hairs, which become enlarged at the tip, and finally 

 develop into the sporangia. Meanwhile the edge of the leaflet 

 is bent down and under so as to make a longitudinal band of 

 thin tissue composed of epidermis known as the oider veil or 

 indusium (Fig. 64, o.i). A similar thin sheet of epidermis 

 grows down from the under side of the leaf, and passing out- 

 wards to meet the former, constitutes the inner veil or true 

 indusium (Fig. 64, B^ i.i). 



In the Y-shaped space thus formed the sporangia are de- 

 veloped. 



A superficial (epidermal) cell enlarges and becomes divided into a 

 proximal (basal) cell and a distal (apical) cell (Fig. 65, a). The former de- 

 velops into the iwiuve pedicel or stalk of the sporangium ; the latter gives 

 rise to the head or capsule within which the spores are formed (cf. Fig. 63>. 

 The pedicel arises from the original pedicel-cell by continued growth and 

 subdivision until it consists of three rows of cells somewhat elongated. 

 The rounded capsule-cell is next transformed by four successive oblique 

 divisions into four plano-convex "parietal cells" and a tetrahedral central 

 cel>, the archesporium, enclosed by the others. The capsule-cell is thus 

 divided by three planes inclined at about 120° (Fig. 65, 6, c). A fourth 

 (Fig. 65, d, e) passes nearly parallel to the top of the capsule and cuts off 



