540 



NA TURE 



LOcTOHKR 1, 1896 



has been successfully accomplished (Fucus), Prof. Farmer and 

 Mr. Williams find exactly the reverse ; the plant has throughout 

 ihe/u// number of chromosomes ; reduction first take.s place in 

 the oiigoniuin, immediately before the maturation of the ova, 

 and on sexual fusion the full number is restored, to persist 

 throughout the vegetative life of the plant, fuciis is, no 

 doubt, a long way off the direct line of descent of Archegoniata;, 

 but still it is a striking fact that the only direct evidence we 

 have goes dead against the idea that the sexual generation (and 

 who could call a Fucus-plant anything else but sexual ?) neces- 

 sarily has the reduced number of chromosomes. This fact is 

 indeed a rude rebuff to deductive morphology. 



I am disposed to regard the different number of chromosomes 

 in the two generations observed in certain cases among 

 Archegoniat^ not as a primitive but as an acquired phenomenon, 

 perhaps correlated with the definiteness of alternation in the 

 Archegonialte as contrasted with its indefiniteness in Thallo- 

 phytes. In Fucus, in flowering plants, and in animals the soma 

 or vegetative body has the full number of chromosomes. With 

 these the sporophyte of the Archegoniata; agrees ; it is the 

 oiiphyte which appears to be peculiar in possessing the half- 

 number, so that if the evidence points to intercalation at all, it 

 would seem to suggest that the ociphyte is the intercalated 

 generation — obviously a reductio ad absiirditm. I do not think 

 we are as yet in a position to draw any morphological con- 

 clusions from these minute histological differences, interesting 

 as they are. 



The question how the number of chromosomes is kept right 

 in cases of apospory and of apogamy is obviously one of great 

 interest, and I am glad to say that it is receiving attention from 

 competent observers. 



Sexu.^i.ity of Fungi. 



Only a few years ago De Bary's opinion that the fruit of the 

 ascus-bearing B'ungi is normally the result of an act of fertilisa- 

 tion was almost universally accepted, especially in this country. 

 Although the presence of sexual organs had only been recorded 

 in comparatively few cases, and the evidence for their functional 

 activity was even more limited, yet the conviction prevailed that 

 the ascocarp is at least the homologue of a sexually produced 

 fruit. The organ giving rise to the ascus or asci was looked 

 upon as homologous with the oogonium of the Peronosporeae, 

 the supposed fertilising organ either taking the form of an 

 antheridial branch as in that group, or, as observed by Stahl in 

 the lichen Collcnia, giving rise to distinct male cells, or sper- 

 matia. More recently there has been a complete revolution of 

 opinion on this point, and a year ago or less most botanists 

 probably agreed that the question of the sexuality of the 

 Ascomycetes had been settled in a negative sense. This change 

 was due, in the first place, to the influence of Brefeld, who 

 showed, in a great number of laborious investigations, that the 

 ascus-fruit may develop without the presence of anything like 

 sexual organs ; while Miiller proved that the supposed male 

 cells of lichens are in a multitude of cases nothing but conidia, 

 capable of independent germination. 



The view thus gained ground that all the higher Fungi are 

 asexual plants, fertilisation only occurring in the lower forms, 

 such as the Peronosporea; and Mucorineoe, which have not 

 diverged far from the algal stock. The ascus, in particular, is 

 regarded by this school as homologous with the asexual spor- 

 angium of a Aluio?: This theory has been brilliantly expounded 

 in a remarkable book by Von Tavel, which we cannot but 

 admire as a model of clear morphological reasoning, whether 

 its conclusions be ultimately adopted or not. 



Still, it must be admitted that the Brefeld school were rather 

 apt to ignore such pieces of evidence as militated against their 

 views, and consequently their position was insecure so long as 

 those hostile posts were left uncaptured. 



Quite recently the whole question has been reopened by the 

 .striking observations of Mr. Harper, an American botanist 

 working at Bonn. 



Zopf, in 1890 (Die Pilze, " Schenk's Handbuch der Botanik," 

 Bd. iv. p. 341), pointed out that up to that tiine it had not been 

 possible in any Ascomycete to demonstrate a true process of 

 fertilisation by strictly scientific evidence, namely, by observing 

 the fusion of the nuclei of the male and female elements. Exactly 

 the proof demanded has now been afforded by Mr. Harper's 

 observations, for in a simple Ascomycete, Sphicrotheca castagitei, 

 the parasite causing the hop-mildew, he has demonstrated in a 

 manner which appears to be conclusive the fusion of the nucleus 



NO. 1405, VOL. 54] 



of the antheridium with that of the ascogonium {lierichte der 

 deiitsihen hot. Gesellschaft, vol. xiii., January 29, 1896). It is 

 impossible to evade the force of this evidence, fur the fungus in 

 question is a ])erfectly typical Ascomycete, though exceptionally 

 simple, in .so far as only a single ascus is normally produced 

 from the ascogonium. It is unnecessary to point out how 

 important it is that Mr. Harper's observations should be con- 

 firmed and extended to other and more complex members of 

 the order. In the mean time the few who (unlike your 

 President) had not bowed the knee to Brefeld may rejoice ! 



It is impo.ssible to pursue the various questions which press 

 upon ones mind in considering the morphology of the Fungi. 

 The occurrence not only of cell-fu.sion, but of nuclear fusion, 

 apart from any definite sexual process, now recorded in several 

 groups of Fungi, urgently demands further inquiry. Such 

 unions of nuclei have been observed in the basidia of Agarics, 

 the teleutospores of Uredine*, and even in the asci of the 

 A.scomycetes. That such a fusion is not necessarily, as 

 Dangeard {Le Botaniste, vols. iv. and v.) has supposed, of 

 a sexual nature, seems to be proved by the fact that it occurs in 

 the young ascus of Spharotheca long after the true act of 

 fertilisation has been accomplished. It is possible, however, 

 that these phenomena may throw an important side-light on the 

 significance of the sexual act itself. 



Another question which is obviously opened up by the new 

 results is that of the homologies of the a.scus. The observations 

 of Lagerheim (" Pringsheim's Jahrbuch f. Wiss Bot.," 1892), 

 on Dipodascus point to the sexual origin of a many-spored 

 sporangium not definitely characterised as an ascus. On the 

 other hand, not only sporangia, but true asci are known to arise 

 in a multitude of cases direct from the mycelium. It is of 

 course possible that as regards the asci these are cases of 

 reduction or apogamy ; on the other hand, it is not wholly 

 impossible that the asci may turn out to be really homologous 

 with a sexual sporangia, even though their development may 

 often have become associated with the occurrence of a sexual 

 act. However this may be, there is at present no reason to 

 doubt that a very large proportion of the Fungi are, at least 

 fnnctionally, sexless plants. 



Chalazog.\.my. 

 Among the most striking results of recent years bearing on 

 the morphology of the higher plants, Treub's discovery of the 

 structure of the ovule and the mode of fertilisation in Castiarina 

 must undoubtedly be reckoned. The fact that the pollen tube 

 in this genus does not enter the micropyle, but travels through 

 the tissues of the ovary to the chalaza, thus reaching the base of 

 the embryo-sac, was remarkable enough in itself, and when 

 considered in connection with the presence of a large sporo- 

 genous tissue producing numerous embryo-sacs, appeared to 

 justify the separation of this order from other angiospernis. 

 Then came the work of Miss Benson in Fngland, and of 

 Nawaschin in Russia, showing that these remarkable pecu 

 liarities are by no means confined to Casual iiia, but extend 

 also in various modifications to several genera cif the Cupulifer.v 

 and Ulmacea;. They are not, however, cnnsiant throughout 

 these families, so that we are no longer able tc > attach to these 

 characters the same fundamental systematic importance which 

 their first discoverer attributed to them. It is remarkable, 

 however, that these departures from the ordinary course of 

 angiospermous development occur in families some of which 

 have been believed on other grounds to be among the most 

 primitive Dicotyledons. 



Evidence of Descent derived from F'ossil Botany. 



At the beginning of this Address I spoke of the importance of 

 the comparatively direct evidence afforded by fossil remains as 

 to the past history of plants. It may be of interest if I endeavour 

 to indicate the directions in which such evidence seems at 

 present to point. 



It was Brongniart who in 1828 first arrived at the great 

 generalisation that " nearly all of the plants living at the most 

 ancient geological epochs were Cryptogams " (Williamson, 

 " Reminiscences of a Yorkshire Naturalist," 1S96, p. 198), a 

 discovery of un.sur]iassed importance for the theory of evolution, 

 though one which is now so familiar that we ain.ost lake it for 

 granted. Those palaeozoic plants which are not Cryptogams 

 are (lymnosperms, for the angiospermous flowering plants only 

 make their appearance high up in the secondary rocks. Even 



