300 



COLLEGE BOTANY 



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

 ginal cells for abo'Ut two-thirds of the circumferemces are small 

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

 When mature the absOirption of water by and evaporation f ro^m 

 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 Gam.etophytes. 



The Differemiiaiion 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 

 microsporangia and others macrosporangia ; the former being 

 known as microsporophyUs and the latter as macrosporophylls. 

 The microsporangia bear microspores which produce male 



