November i i, 1897] 



NA TURE 



45 



portions of the plant which is inherited from earlier times. 

 Rather when spore-production appears on the sporophyte could 

 this process be looked upon as a reversion to the primary office 

 of the sporophyte, so that in spore-production of the higher 

 plants we may have a constantly recurring reversion to a 

 process which in the remote past was the sole function 

 of this phase of the plant. In this way might be ex- 

 plained those cases where sporangia occur on the normal 

 foliage leaf of Botrychium, and some peculiar cases which 

 I have observed in Osinunda cinnainomea. In some of 

 the examples of this species it would appear that growth 

 of the leaf was marked by three different periods even after the 

 fundament was outlined ; the first, a vegetative ; second, a spore- 

 producing ; and third, a vegetative again ; for the basal portions 

 of the leaf are expanded, the middle portions spore-bearing, the 

 passage into the middle portions being gradual, so that many 

 sporangia are on the margins of quite well-developed pinnae. 

 These gradations of the basal part of the leaf, and their relation 

 to the expanded vegetative basal portion, showing that the 

 transition here has been from partially formed foliage leaf to 

 sporoph) 11 after the fundament was established, and later the 

 increments of the vegetative part from the middle towards the 

 terminal portion, shown by the more and more expanded con- 

 dition of the lamina and decreasing sporangia, indicate that 

 vegetative forces are again in the ascendency. This suggests 

 how unstable is the poise between the vegetative leaf and 

 sporophyll in structure and function in the case of this species. 



For two successive years I have endeavoured by experiment to 

 produce this transformation in Osmittida cinuamomea, but thus 

 far without sufficiently marked results. The stem of the plant 

 is stout, and this, together with the bases of the leaves closely 

 overlapping, contain considerable amounts of stored nutriment 

 which make it difficult to produce the results by simply cutting 

 off the foliage leaves. The fact that these transformations are 

 known to occur where fire has overspread the ground, and, as I 

 have observed, where the logging in the woods seriously injured 

 the stools of the plant, it would seem that deeper-seated injuries 

 than the mere removal of foliage leaves would be required to 

 produce the transformation in this species. It may be that such 

 injury as results from fire or the severe crushing of the stools of 

 the plant would be sufficient to disturb the equilibrium which 

 existed at the time, that the action of the correlative forces is 

 changed thereby, and there would be a tendency for the 

 partially developed foliage leaves to form sporangia, then when 

 growth has proceeded for a time this balance is again changed. 



The theory that the foliage leaves of the sporophyte have 

 been derived by a process of sterilisation, and that the trans- 

 formation of sporophylls to foliage leaves, in an individual, 

 indicates the mode of progress in this sterilisation, does not 

 necessarily involve the idea that the sporophyll of any of the 

 ferns, as they now exist, was the priir.ary form of the leaf in that 

 species ; and that by sterilisation of some of the sporophylls, 

 4he present dimorphic form of the leaves was brought about. 

 The process of the evolution of the leaf has probably been a 

 gradual one, and extends back to some ancestral form now 

 totally unknown. One might differ from Prof. Bower ; the 

 ^ examples selected by him to illustrate the course of progress 

 from a simple and slightly differentiated sporophyte to that 

 , exhibited in the various groups of the Pttridophyta. But it 

 seems to me that he is right in so far as his contention for the 

 evolution of vegetative and assimilatory members of the sporo- 

 phyte, can be illustrated by a comparison of the different degrees 

 of complexity represented by it in different groups, and that this 

 illustrates the mode of progress, as he terms it, in the sterilisa- 

 tion of potential sporogenous tissue. 



On this point it appears that Prof. Bower has been unjustly 

 criticised. The forms selected to illustrate his theory were 

 chosen not to represent ancestral forms, or direct phylogenetic 

 lines, but solely for the pnrpose of illustrating the gradual 

 transference of spore-bearing tissue from a central to a 

 peripheral position, and the gradual eruption and separation of 

 spore-bearing areas, with the final sterilisation of some of these 

 outgrowths. 



To maintain that in phylogeny the sporophyll is a transformed 

 foliage leaf, would necessitate the predication of ancestral 

 plants with only foliage leaves, and that in the case of these 

 plants the vegetative condition of the sporophyte was the 

 primary one, spore production being a later developed function. 

 Of the forms below the Pteridophyta, so far as our present 

 evidence goes, the sporophyte originated through what Bower 



calls the gradual elaboration of the zygote. All through the 

 Bryophyta wherever a sporophyte is developed, spore production 

 constantly recurs in each cycle of the development, and yet 

 there is no indication of any foliar organs on the sporophyte. 

 The simplest forms of the sporophyte contain no assimilatory 

 tissue, but in the more complex forms assimilatory tissue is 

 developed to some extent, showing that the correlative forces 

 which formerly were so balanced as to confine the vegetative 

 growth to the gametophyte and fruiting to the sporophyte, are 

 later changing so that vegetative growth and assimilation are 

 being transferred to the sporophyte, while the latter still retains 

 the function of spore production, though postponed in the 

 ontogeny of the plant. 



If we cannot accept some such theory for the origin of 

 sporophylls and foliage leaves, by gradual changes in potential 

 sporogenous tissue, somewhat on the lines indicated by Bower, 

 it seems to me it wculd be necessary, as already suggested, to 

 predicate an ancestral form for the Pteridophyta in which spore 

 production was absent. That is, spore production in the 

 sporophyte of ancestral forms of the Pteridophyta may never 

 have existed in the early period of its evolution,' and spore 

 production may have been a later development. But this, 

 judging from the evidence which we have, is improbable, since 

 the gametophyte alone would then be concerned in transmitting 

 hereditary characters, unless the sporophyte through a long 

 period developed the gametophyte stage through apospory. 

 Bower says, in taking issue with Goebel's statement that the 

 experiments on Onoclea prove the sporophyll to be a transformed 

 foliage leaf: "I assert, on the other hand, that this is not 

 proved, and that a good case could be made out for priority of 

 the sporophyte ; in which event the conclusion would need to 

 be inverted, the foliage leaf would be looked upon as a 

 sterilised sporophyll. This would be perfectly consistent with 

 the correlation demonstrated by Prof. Cioebel's experiments, as 

 also with the intercalation of a vegetative phase between the 

 zygote and the production of spores.'' In another place he says :, 

 '■ To me, whether we take such simple cases as the Lycopods or 

 the more complex case of the Filicinece, the sporangium is not 

 a gift showered by a bountiful Providence upon pre-existent 

 foliage leaves : the sporangium, like other parts, must be looked 

 upon from the point of view of descent ; its production in the 

 individual or in the race may be deferred, owing to the inter- 

 calation of a vegttative phase, as above explained ; while, in 

 certain cases at least, we probably see in the foliage leaf 

 the result of the sterilisation of sporophylls. If this be so, 

 much may be then said in favour of the view that the appearance 

 of sporangia upon the later formed leaves of the individual is a 

 reversion to a more ancient type rather than a metamorphosis of 

 a progressive order." 



As I have endeavoured to point out in another place, if a 

 disturbance of these correlative processes results in the trans- 

 ference of sporophyllary organs to vegetative ones on the 

 sporophyte, " why should there not be a similar influence 

 brought to bear on the sporophyte, when the same function 

 resides solely in the gametophyte, and a disturbing element of 

 this kind is introduced? To me there are convincing grounds 

 for believing that this influence was a very potent — though not 

 the only — one in the early evolution of sporophytic assimilatory 

 organs. By this I do not mean that in the Bryophyta, for 

 example, injury to the gametophyte would now produce distinct 

 vegetative organs on the sporophyte, which would tend to 

 make it independent of the gametophyte. But that in the 

 bryophyte-like ancestors of the pteridophytes an influence of 

 this kind did actually take place, appears to me reasonable. 



" In the gradual passage from an aquatic life, for which the 

 gametophyte was better suited, to a terrestrial existence for 

 which it was unadapted, a disturbance of the correlative pro- 

 cesses was introduced. This would not only assist in the 

 sterilisation of some of the sporogenous tissue, which was taking 

 place, but there would also be a tendency to force this function 

 on some of the sterilised portions of the sporophyte, and to 

 expand them into organs better adapted to this office. As 

 eruptions in the mass of sporogenous tissue took place, and 

 sporophylls were evolved, this would be accompanied by the 

 transference of the assimilatory function of the gametophyte to 

 some of these sporophylls." 



Because sporophytic vegetation is more suited to dry land 

 conditions than the gametophytic vegetation, it has come to be 

 the dominating feature of land areas. Because the sporophyte 

 in the Pteridophyta and Spermatophyta leads an independent 



NO. 1463. VOL. 57] 



