326 ABNORMALITIES OF THE LIFE-CYCLE [CH. 



to 1 6 chromosomes takes place as usual in the spore-mother-cells, in M. 

 Drummondii the megasporangia have two types of spore-mother-cells. The 

 one type is normal in number, and shows reduction : the other type is pro- 

 duced in smaller number in the sporangia, for instance there may be only 

 four spore-mother-cells in place of the normal i6. These on division have 

 diploid nuclei, and the interesting fact is that their diploid state does not 

 divert the resulting spores from the usual form and structure. Since the apo- 

 gamous plants produce both diploid and haploid spores, it is not surprising 

 that both apogamous and sexual prothalli should be produced on their 

 germination, and it follows that among the representatives of the species 

 there will be individual cycles completed without any change of chromosome- 

 number. Certain cycles will thus be diploid throughout ; the spore may be 

 diploid, also the prothallus which springs from it, and even the Qgg itself. 



Almost simultaneously the chromosome-cycle of a number of other 

 abnormal Ferns was being worked out by Farmer and Miss Digby {Ann. 

 of Bot. 1907, p. 161). Among these they found that Athyrmm Filix-foemina, 

 var. clarissima, Bolton, corresponds to the abnormal condition of Marsilia 

 Driinnnondii in being diploid throughout. The prothalli are readily produced, 

 usually from the sorus, but occasionally from the apex of the pinnules. 

 They are very fertile, bearing sexual organs freely : but there is no fertilis- 

 ation, nor any migration of nuclei. There is thus neither reduction nor 

 doubling of chromosomes, and the new sporophyte, so far as observed, 

 " invariably arises from the oosphere." Scolopendrium vulgare, var. crispum 

 Drmnmoiidiae is also like these in all essential details. But Athyriuni Filix- 

 foemina, var. clarissima^ Jones, differs sharply even from A. F.-f., var. claris- 

 sima, Bolton, in the fact that the embryo arises from the prothallus by 

 apogamous budding, and not from a diploid ovum. Apart from these details, 

 in all of the above the life-cycle is diploid throughout, including even the 

 gametophyte, in which there is no change of external character to be seen. 



The further question will then present itself whether the converse is 

 possible, or has been observed, viz. that the normally diploid sporophyte 

 may be haploid, having only the reduced number of chromosomes. A 

 reasonably probable case has been established by Farmer and Miss Digby 

 in Lastraea pseudo-vias, var. cristata apospora, Druery. The detached leaf 

 produces aposporous prothalli from its margin or surface, which bear oc- 

 casional antheridia, and the sporophyte is produced apogamously. The 

 chrorriosome-number in the prothallus is about 60: in the embryo the 

 number varies round mean countings of 60 and 78. No migration of nuclei 

 has been observed, nor is there any reduction in the whole cycle. The facts 

 suggest that the gametophyte-character has been impressed upon the sporo- 

 phyte, the converse in fact of what has been seen in the varieties oi AtJiyrium, 

 and in Marsilia. A similar condition was very fully made out by Yama- 



