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. — Spliagnumsp. A,B, 

 young protonemata; C, older pro- 

 tonema with leafy bud, k; r, mar- 

 ginal rhizoids. (After Campbell.) 



