300 



gether. The spores are borne within this structure. The mar- 

 ginal cells for about two-thirds of the circumferences are small 

 and thick-walled, while the others are large and thin-walled. 

 When mature the absorption of water by and evaporation from 

 this ring of cells is uneven, resulting in a rupturing of the spo- 

 rangium and a scattering of the spores. These spores germinate 

 and produce new prothallia or Grametophytes. 



The Differentiation of the Sporophylls, Sporangia, and 

 Spores. It will be readily seen that the leaves (or fronds) of 

 the ferns serve two very distinct functions, photosynthesis and 



FIG. 147. 



FIG. 145. 



FIG. 149. 



FIG. 145. Young fern leaf showing method of unrolling. 

 FIG.. 146. Part of fern leaf showing sori or fruit clusters. 

 FIG. 147. Part of fern leaf showing sori with indusium. 

 FIG. 148. Sporangium from fern sorus. 

 FIG. 149. Fern spores from sporangium. 



bearing sporangia which contain non-sexual spores. The photo- 

 synthetic function is the same in all chlorophyll-bearing plants. 

 The leaves of some ferns perform both functions. A fern leaf 

 which bears sporangia is known as a sporophyll. The leaves of 

 some ferns are so differentiated that certain parts serve for 

 bearing sporangia only. In other species certain leaves serve 

 one function and others serve the other function. Furthermore, 

 in some species of the Pteridophytes certain sporophylls bear 

 microsporangw, and others macrosporangia ; the former being 

 known as microsporopkytts and the latter as macrosporophylls. 

 The microsporangia bear micros pores which produce male 



