PERIODICITY IN DEVELOPMENT. II 



359 



Fig. 107. Polypodium vulgare. A, 

 mature, ff, empty antheridium; , cell 

 of the prothallus ; / and 2 parietal cells ; 

 3, cover -cells (x 240). C, a motile 

 sperm. Z>, the same fixed by iodine 

 solution ( x 540). From the Bonn Text- 

 book. 



abnormal phenomena which occur in many ferns under conditions which have 

 not as yet been fully studied, viz. the phenomena termed apospory and apogamy 

 (for literature see GOEBEL, 1898-1901, p. 430). By apogamy is meant the 

 development of the sporophyte, or part of it, the sporangium, as a vegetative 

 outgrowth of the prothallus, without any intervention of the fertilized ovum. 

 Again when prothalli arise not from spores but 

 from sterile sporangial - cells, or from general 

 vegetative- cells of the sporophyte, we speak of 

 the phenomenon as apospory. Apogamy and 

 apospory show that the characters of the sporo- 

 phyte, as also of the gametophyte, lie latent in 

 every cell of the fern, and that the changes 

 leading to the formation of the other generation 

 do not occur first in the egg-cell or in the spore. 

 Apospory and apogamy lead us to hope that it 

 may yet be possible to discover the more im- 

 mediate conditions that determine the rhythm of 

 alternation so that we may then be able pos- 

 sibly to alter it at will. 



Another variation from the regular alterna- 

 tion of generation occurs much more frequently 

 than apospory and apogamy. Many ferns have 

 in addition to spores and sexual cells other repro- 

 ductive organs which may be termed accessory. 

 These may occur on the sporophyte or on the 

 prothallus, but in neither case do they form stages in the alternation ; one 

 variety renews the sporophyte, the other the gametophyte (BowER, 1887). 

 The regularity in the alternation of generations may in this way be destroyed 

 and each generation may go on repeating itself directly an indefinite number 

 of times. Doubtless careful investigation may show that external factors 

 often determine whether the typical or the accessory reproductive organs 

 shall be produced in any special case. 



If we now glance at the life-cycle of the other Pteridophyta and compare 



them with that of the 



ferns in the narrower sense 



of the term, we find that 



we are able to identify in 



all of them the same two 



generations, but not al- 

 ways with equal clearness. 



The gametophyte in very .^W^SX_-^ Y^fflg^A JL* 



many forms reaches only 



a very limited size, and is 



finally entirely enclosed 



within the spore. The 



wall of the spore opens at 



one end, in the one case 



to allow the sperms to 



escape, in the other to 



afford a means of entry 



for the sperm to the archegonium and to the ovum. Concomitantly with 



these reductions in size of the prothallus, further modifications occur in its 



historical development. While both types of sexual organs are or may be borne 



on the prothalli of ferns, in other types the prothalli are unisexual, and it depends 



on the character of the spore and the sporangium whether the prothallus 



Fig. 108. Polypodium vulgare. A, immature archegonium; K\ 

 neck canal-cells ; A"", ventral canal-cells ; 0, ovum. B, mature and open 

 archegonium (x 240). From the Bonn Textbook. 



