324 



ABNORMALITIES OF THE LIFE-CYCLE 



[CH. 



^i"' 



had already been indicated by Overton, was welcomed as giving a new 

 precision to the facts of alternation. Observation of the details has shown 

 in a vast number of normal life-histories that 

 the chromosome-distinction between the 

 generations is a just one. It applies equally 

 to certain Algae, to the Archegoniatae, and 

 to Flowering Plants, and may be accepted 

 as a state to which a very large proportion 

 of living plants definitely adhere. The recog- 

 nition of this normal chromosome-cycle at 

 once concentrated attention more critically 

 than ever upon those abnormalities which 

 are included under the terms apogamy and 

 apospory : and the question as to the nuclear 

 facts in such cases was recognised as of the 

 utmost importance in their interpretation. 

 On the other hand, these facts must neces- 

 sarily influence any true estimate of the 

 value of the chromosome-cycle itself in rela- 

 tion to evolutionary theory. 



Already in 1898 Lang had observed in 

 prothalli of Scolopendriuni the frequent 



occurrence of two nuclei in a single cell of the tissue bordering on the 

 change from gametophyte to sporophyte (Fig. 300). More detailed observa- 

 tions have since been made on other apogamous Ferns : for instance on 

 Nephrodiuin pseudo-mas, va.x. polydactyluin (Farmer, Moore, and Miss Digby, 

 Proc. Roy. Soc. Vol. Ixxi, p. 453). Examining young prothalli before any apo- 

 gamous growths began to manifest themselves, it was found that certain 

 cells contained two nuclei: when that was so a neighbouring cell was seen to 

 be without a nucleus, and cases were found where the passage of the nucleus 

 through a hole in the cell-wall was actually in progress (Fig. 302). Fusion of 

 the two nuclei followed, and the process was regarded as a kind of irregular 

 fertilisation. On their division the nuclei of the apogamous growth thus 

 initiated show evidence of doubling of the number of chromosomes, just 

 as happens in the normal post-sexual stage. On the other hand, meiosis 

 occurs in the spore-mother-cells of this Fern, and the spores germinate to 

 form a haploid prothallus. Thus the chromosome-cycle is essentially the 

 same as in Dryopteris Filix-j/ias, except for the substitution of an irregular 

 fusion in place of the normal syngamy : and the cytological criterion between 

 the two generations holds good. 



The apparently simple cycle oi Marsilia Druvtmondii, A. Br. was worked 

 out by Strasburger in 1907 {Flora, Bd. xcvii, p. 123). It was found to depart 



Fig. 301. Ceratopteris thalictroides. Re- 

 generation from a primary leaf, show- 

 ing an intermediate state between pro- 

 thallus and leaf. ^ = antheridia; 

 j^^' = stoma. (After von Goebel.) 



