THE ANCIENT FERNS 



opening, the neck cells of the topmost tier generally seem to loosen from each other and 

 are thrown off, after which the neck closes whether fertilization has occurred or not. An 

 early stage in the process of fertilization is illustrated in Fig. 266a, but no developing 

 embryos were seen. The signs of nuclear fusion in the first few days after insemination 

 were, however, sufficiently clear to leave little doubt that the gametes are fully func- 

 tional.* 



This fact is of some importance, for otherwise the question might have been raised as 

 to whether the nuclear condition subsequently found in the sporophyte is compatible with 

 effective sexuality. The reader will by now have become somewhat acclimatized to a 

 record of high chromosome numbers, especially after seeing Equisetum and Tmesipteris 

 (Chapters 13 and 14). In Ophioglossum vulgatum, however, we have an even more ex- 

 treme case. As Figs. 262 b and 263 b will show, the chromosomes are sm^aller than those 

 of 0. lusitanicum and about twice as numerous. There is no sign of multivalents, so that 

 we need not ask whether this is merely a polyploid strain. The number is difficult to 

 assess with final accuracy, but allowing the maximum margin of uncertainty that the 

 observations require, it may be said with confidence that in 0. vulgatum the gametic 

 chromosome number is not less than 250 nor more than 260, and that the correct figure 

 lies somewhere between these limits. This is the highest chromosome number yet dis- 

 covered in a wild species in the plant kingdom, for it means that there are more than 500 

 chromosomes per cell in the sporophytic tissue. 



It is so probable from these approximate figures that 0. vulgatum has exactly twice the 

 chromosome number of 0. lusitanicum, that this may be safely assumed without further 

 demonstration, and it is here that the range of possible numbers already mentioned for 

 the latter species becomes of importance. We are clearly dealing with the upper mem- 

 bers of a polyploid series, the lower members of which are unknown, and we cannot at 

 the moment diagnose the fundamental chromosome number until either the lower mem- 

 bers are found or greater accuracy can be introduced into our study of the higher ones. 

 If, for example, the correct figures for the two British species could be shown to be 

 exactly 128 and 256 respectively, we should be unquestionably dealing with a polyploid 



* In an attempt to observe the cytological details of fertilization a number of small prothalli were 

 inseminated on 21 June 1940, and fixed at intervals thereafter. The presence of opened archegonia was 

 checked in each case by microscopic examination at the time of insemination. The first specimen, which 

 was fixed within an hour of the start of the experiment, namely, at 7 p.m. on 21 June, contained the 

 archegonium with the still open neck canal photographed in Fig. 266a. Three hours later a second speci- 

 men showed an archegonium with a male nucleus somewhat swollen but definitely inside the egg. On 

 the following day at i p.m. a third specimen was fixed, which on sectioning showed a female nucleus with 

 a crescentic reticulated mass spread partly over its surface, which resembled very closely a fertilization 

 stage seen in Equisetum sylvaticum, so that it can safely be interpreted in the same way. Unfortunately, this 

 specimen had been cut by the microtome knife rather inconveniently, the centre of the crescent being in 

 one section and the two arms in the next, so that a convincing photograph is difficult to obtain. A fourth 

 specimen fixed at the end of a week showed no trace of unabsorbed male nuclei but also no trace of 

 division of the egg. The only difference between this and the behaviour oi Equisetum or Dryopteris, in both 

 of which I have successfully followed the details of fertilization, is the much greater slowness of Ophio- 

 glossum. The late fusion stage described after 18 hours is reached in three in Equisetum, and in Dryopteris 

 the first division of the egg occurs after a week. It is therefore probable that had observations been con- 

 tinued for longer the cleavage stages might have been seen. As it is, there is only proof of nuclear fusion. 



268 



