XVI] THE CHROMOSOME-CYCLE 327 



nouchi for NepJirodiiiiii violle, in which normally the chromosome-cycle is 

 based upon the gametophyte-number of 64 or 66. When apogamy occurs 

 a sporophytic bud appears by direct vegetative growth, whose cells on mitosis 

 still give the gametophyte-number of 64 or 66. A further example is recorded 

 with all necessary detail by Steil for Nephrodiiini hirtipes, Hk. in which the 

 prothallus arises by germination of haploid spores : but the gametophyte 

 never produces archegonia : the embryo originates as a vegetative growth 

 from the prothallus. No nuclear migrations or fusions have been observed, 

 and the sporophyte retains the haploid number of 60 to 65 chromosomes. 



It seems unnecessary to multiply instances of such irregularities of the 

 chromosome-cycle, or to give an exhaustive abstract of the now voluminous 

 literature. Abundant references are quoted in the memoirs here cited. From 

 what has been stated above it is clear that the gametophyte may be diploid, 

 though normally it is haploid : and that the sporophyte may be haploid, 

 though normally it is diploid : in neither case does the cytological difference 

 appear to affect the form or structure of the gametophyte or sporophyte in 

 question. The general conclusion from these facts may be stated substantially 

 in the terms of Prof. Farmer and Miss Digby (278, p. 197): that no necessary 

 relation exists between periodic reduction in the number of chromosomes 

 and the alternation of generations. Therefore the problem of alternation 

 and its nature must be settled by an appeal to evidence other than that 

 derived from the facts of meiosis. That, so far as the Archegoniatae are 

 concerned, alternation is normally associated with meiosis on the one hand 

 and syngamy on the other no one will dispute. But now that it has been 

 shown that no necessary connection exists between alternation and the usual 

 nuclear cycle, it is scarcely to be wondered at if the presumed correlation 

 should often break down in complex cases. So long as the exceptions were 

 of rare occurrence they have been regarded as monstrosities or abnormalities. 

 But now that records of them have become common, they are to be looked 

 upon as proofs of current physiological instability. Each may be held to 

 result from an individual break-away from the normal course of events. 

 There is no reason to hold that such a break-away represents any state 

 which has had a settled place in the previous history of the individual or of 

 the race. In many cases, or even in most of them, the source may probably 

 lie in hybridisation of gametes in some sense incompatible, so as to prevent 

 or alter the readjustment of the nuclear mechanism involved in meiosis. Such 

 hybridisation is always liable to occur in promiscuous growths of prothalli 

 on moist soil, with casual juxtaposition of those derived from various sources ; 

 and a method of attraction determining the movements of the spermato- 

 zoids, common to several prothalli, would encourage it. 



The instances quoted suggest that within certain limits each plant is a 

 law to itself; but in certain points those limits are binding. Sexuality and 



