TRANSACTIONS OF SECTION K. 1055 



two important classes of facts, whicli have been regarded as affording' more direct 

 evidence in favour of the antithetic and homologous theories respectively. These 

 are the cytological differences between the two generations, and the deviations from 

 the normal life history known as apospory and apogamy. 



The first of tliese will only be mentioned. The existence of the double number 

 of chromosomes, which results from the sexual fusion, in the nuclei of the sporo- 

 phyte, throughout Bryophyta, Ferns, and the higher plants, certainly appears to 

 lend support to the view that the sporophyte is an interpolated stage in the 

 life history. From the cytological point of view the intercalation is between the 

 doubling of the number of chromosomes by the sexual fusion and the reduction in 

 number in the spore mother cells. Facts are wanting as to the nuclear changes in 

 Thallophytes, and also in apogamy and apospory. 



These latter phenomena are the last element in the problem that can be referred 

 to at length. We saw that in the case of the alternation of clearly homologous 

 generations in the Thallophyta it had been shown that the assumption of the sexual 

 or asexual form depends on the external conditions. This experimental study 

 needs to be extended to the rudimentary sporophytes of the Green Algje, but with 

 regard to these it is already known that in (Edogo)iiuvi the fertilised ovum may 

 grow out directly into a vegetative plant, instead of dividing into spores. In the 

 Archegoniata>. tliis complete substitution of one generation for another is not known 

 to occur ; no variations in the external conditions are known to induce a Fern spore 

 to develop into a Fern plant, or the fertilised ovum to give rise directly to a prothallus. 

 But the facts as to the direct development of the one generation from the tissues of 

 the other, and the existence of structures which may fairly be described as inter- 

 mediate between gametophyte and sporophyte are sufBciently striking. 



The main facts with regard to apospory, the vegetative origin of the gametophyte 

 from the tissues of the sporophyte, are briefly these. In Mosses cut portions of the 

 seta or capsule have been induced to give rise to protonemal filaments ; in one 

 case this is known to have occurred in nature while the capsule was still attached 

 to the Moss plant. In a number of Ferns the production of prothalli from the 

 sporangia, the placenta, the surface of the leaf or the leaf margin, takes place. In 

 Scolopendrium vulgare and Nephrodium Filix-'mas varieties are known in which 

 the first leaves of the young sporophyte exhibit this capability of producing prothalli. 

 The causation of this phenomenon is still obscure. In a number of cases sporal 

 arrest has been shown with probability to be of importance, notably in the case of 

 Onoclea, in which apospory occurred on fertile leaves which had been experimentally 

 induced to assume the vegetative form. Further, the fact that conditions of life 

 favourable to the gametophyte, such as laying the fronds on damp soil, determine 

 the growth of prothalli from the tissues of some aposporous Ferns may be 

 mentioned. As to the weight to be attached to apospory it must be borne in mind 

 that the phenomenon is little more wonderful than the fact of the spore, a cell 

 isolated from the sporophyte, producing a prothallus. Here, as in the case of 

 apogamy, the investigation of the cytological details is urgently needed. 



The deviations from the normal life history, which are classed as apogamy, 

 may be considered to possess more importance as suggesting the homology of the 

 two generations in the Ferns. Though as yet only known in this group of plants, 

 apogamy has been found in more than twenty species. In some the young Fern- 

 plant arises on the under surface of the prothallus, which in these cases often 

 bears few or no sexual organs. But in cases in which apogamy has been induced 

 the characters of the two generations may be much more intimately blended. 

 Thus tracheides may occur in a prothallus more or less modified in external form! 

 This may grow on as a bud, or may bear isolated members of the sporophyte,' 

 leaves, roots, ramenta, or sporangia. The characters of the two generations are 

 here united in the same individual in a way that at least suggests a oradual 

 transition from gametophyte to sporophyte. 



It is to be hoped that the further study of these deviations from the normal 

 development will lead to their causation being made clear. This may minimise 

 the importance to be attached to them, especially should they be found to depend 

 on a nuclear change. The facts regarding the cytology of these new growths are 



