202 



STRUCTURE AND LIFE HISTORIES 



sporophytes of the ancestors of Sphagnum possessed true 

 stomata and the function of photosynthesis. 



185. Formation of Spores.^ — As the spore-case develops, 

 the inner cells become differentiated into two kinds, one 

 composing the larger part of the tissue, and the other, 

 larger and richer in protoplasm, forming a dome of sporo- 



genous or spore-forming tissue 

 near the upper wall (Fig. 145). 

 From this tissue, spore- 

 mother-cells are developed, 

 and from each of these, by 

 reducing divisions, as in the 

 fern, four spores. 



186. Asexual Reproduc- 

 tion. — While the spores are 

 maturing, a circular groove 

 (annulus) is formed near the 

 apex of the spore-case, and 

 the cells in this zone have 

 thinner walls than those ad- 

 jacent (Fig. 145). At the 

 maturity of the spore-case these cells become dry, and are 

 easily torn apart, thus forming a lid, or operculum, at the 

 summit of the spore-case. The falling away of the 

 operculum affords an opportunity for the scattering of the 

 spores. Under favorable conditions the spores germinate, 

 putting forth a very short, green protonema, as in the case 

 of fern-spores. The tip of the protonema soon broadens 

 out, forming a prothallus, much like that of the fern in 

 shape, but being only one cell thick (Fig. 146). Rhizoids 

 form on , the } under side, and from the margin other 

 threads develop, having chlorophyll, and resembling the 



Fig. 146. — Sfhagnum sp. A, 

 B, young protonemata; C, older 

 protonema with leafy bud, k; r, 

 marginal rhizoids. 



