PTERIDOPHYTA 



263 



The Sporangium 



The formation of spores may not occur for many years after the 

 sporophyte is established. Finally upon certain leaves, sporophylls, 

 which may or may not differ from the foliage leaves, the sporangia are 

 developed. The most generalized type is that of Ophioglossum (Fig. 

 232). Here the sporogenous tissue arises from a hypodermal layer, 

 very much as in Anthoceros, and the distinction between sporoge- 

 nous and sterile cells is not at first evident. The masses of spores 

 are very large, and are discharged through a transverse cleft in the 

 overlying tissue. In other related Ferns e.g. Botrychium, Augi- 

 opteris - the archesporium is also of hypodermal origin, but there is 

 very early an elevation of the superficial tissue so that the sporan- 

 gium projects above the 

 surface of the sporophyll. 



In the most specialized 

 Ferns, the Leptosporangi- 

 ates, the sporangium can 

 be traced back to a single 

 epidermal cell, and the 

 stalked sporangia of these 

 Ferns are most character- 

 istic structures, which are 

 of importance in classify- 

 ing them. 



Spore-formation. The 

 sporogenous cells in all 

 cases divide precisely as 

 in the Bryophytes, into 

 four spores. These may be 

 either tetrahedral in form, 

 or sphere-quadrants result- 

 ing from two successive 

 divisions of the globular 

 mother-cell. Surrounding 

 the mass of sporogeuous 



cells is the tapetum, con- FIG. 228. Polypodium falcatum. 

 sisting of one or more 

 layers of cells, in the 

 Leptosporangiates cut off from the archesporium. The tapetal cells 

 become broken down during the later stages of the spore-development, 

 and the nucleated protoplasm is brought into direct contact with 

 the developing spores, whose growth is doubtless in part due to 

 the activity of the tapetal protoplasm. 



In the typical Ferns, the sporangia are usually in groups, or sori, 



Surface view 

 of a nearly ripe sporaugium (X 175) ; st, sto- 



niiuiu ; r, annul us. 



