TISSUES OF THE LEAF 351 



A sporangium is attached to the leaf by a multicellular 

 stalk (st) and consists of a sac resembling two watch-glasses 

 placed with their concave surfaces towards one another and 

 their edges united by a thick rim (an). The sides are 

 formed by thin flattened cells with irregular outlines, the 

 rim or annulus of peculiarly shaped cells which are thin and 

 broad at one edge (to the left in A), but on the other (to the 

 right) are thick, strongly lignified, and of a yellowish-brown 

 colour. The whole internal cavity is filled with spores 

 (B, sp} having the form of tetrahedra with rounded edges 

 and each consisting of protoplasm containing a nucleus, and 

 surrounded by a double w r all of cellulose. A spore is there- 

 fore, as in mosses, a single cell. 



Each sporangium arises from a single epidermic cell of 

 the leaf. This divides repeatedly so as to form a solid mass 

 of cells, of which the outermost become the wall of the 

 sporangium while the inner are the spore-mother-cells. The 

 latter divide each into four spores, as in mosses (p. 335). 



As the spores ripen, the wall of the sporangium dries, and 

 as it does so the thickened part of the annulus straightens 

 out, tearing the thin cells and producing a great rent through 

 which the spores escape (B). 



When the spores are sown on moist earth they germinate, 

 by the protoplasm, covered by the inner coat, protruding 

 through the ruptured outer coat (c, sp) in the form of a 

 short filament. This divides transversely, forming two cells, 

 the proximal of which sends off a short rhizoid (r/i). The 

 resemblance of this stage to the young protonema of a moss 

 is sufficiently obvious (see Fig. 82, D 1 , p. 334). 



Further cell-division takes place, and before long the 

 distal cells divide longitudinally, a leaf-like body being 

 produced, which is called the prothallus (D). It is at first 



