772 LIFE : OUTLINES OF GENERAL BIOLOGY 



Thus when spores fall from the brown sporangia on the under side 

 of the fern-fronds they develop into prothalli ; and from the fertilised 

 egg-cell of a prothallus there develops the ordinary fern-plant. 

 There is an obvious advantage in having a vigorous sporophyte 

 (what we call "a fern"), with its multitudes of spores which do not 

 require water for their production and propagation, in striking 

 contrast to the small delicate prothallus (or gametophyte), whose 

 egg-cells cannot be fertilised without the presence of external water 

 in which the male cells swim. 



The plasticity and adaptability which we are seeking to em- 

 phasise is illustrated by the occasional production of buds, both 

 on the fern and on the prothallus; and also by the occasional short- 

 cuts, of apogamy and apospory, as when a fern-plant springs 

 directly from tKe prothallus and not from its fertilised egg-cell, or 



Fig. 129. 



Alternation of Generations in Fern Plant. FO, the fertilised ovum, which 

 develops into the asexual sporophyte or ordinary fern plant. A spore 

 (SP) gives rise to the sexual prothallus. 



when a prothallus arises directly on the fern-frond and not from a 

 spore. 



An alternation of parthenogenetic development (from an un- 

 fertilised egg-cell) and ordinary spermic development (from a 

 fertilised egg-cell) is well illustrated among greenflies, water-fleas, 

 wheel-animalcules, threadworms, and flukes; and this kind of 

 alternation is technically called heterogenesis, in contrast to the 

 alternation of asexual and sexual generations, as in hydroids and 

 medusoids (metagenesis). But what happens in ferns and flukes is 

 somewhat between the two types of alternation. 



The general idea may be defined by saying that alternation of 

 generations is the alternate occurrence, in one life-history, of two 

 or more different forms differently produced. We do not wish to 

 get into a tangle, but there is a difficult question that must be 

 asked, though it cannot be at present answered. In flowering plants 

 there is an indubitable alternation of generations, which the genius 

 of Hofmeister disclosed, though it is extraordinarily well concealed 

 or masked. Should this not lead zoologists to be a little more open- 

 minded and inquisitive in regard to the sporadic suggestion that 



