APOGAMOUS FERNS. THE GENERAL PHENOMENON 



and protoplasm and nuclei from this layer closely invest the mass of mother cells until 

 the spores are ripe. At the same time the sporangium enlarges considerably owing to 

 increase of its layer of wall cells, so that during the act of meiosis the mass of tapetum and 

 mother cells appears as if suspended in a cavity far too large for it and which will not 

 be filled until the spores themselves reach their final size. Some of these stages can be 

 seen in Fig. 164 and elsewhere in this chapter and the next. 



Fig. 165. Sections to show very young stages of sporangial development 

 in Cyrtomium falcatum (L.f.) Presl. x 500. For description see text. 



In the apogamous ferns extreme uniformity prevails over the early stages up to the 

 first few synchronized mitoses of the archesporium (Figs. 166, 167 a) after which one 

 of four dififerent things may happen. 



(i) In some sporangia all four successive archesporial cleavages may be completed 

 and sixteen spore mother cells result (Fig. 170a). The proportion of such sporangia 

 varies somewhat from species to species; in some they are abundant and in others 

 extremely rare though they never seem to be entirely absent. Sporangia of this type are 

 of extreme interest from an evolutionary point of view for they display the true pairing 

 homologies of the chromosomes. In most cases, however, they are of no importance for 

 the reproduction of the species and their spores abort. 



(2) The second type of sporangium is the one which is responsible for reproduction. 

 In this the first three archesporial cleavages are normal, but the last division which 



163 11-2 



