714 TRANSACTIONS OF SECTION K. 



case from the spore. Wliile peculiarities may already have been impressed upon 

 it, a metabolic relation with the prothallus still continues, and the developing 

 embryo may be further influenced by tlie latter. The essential of the view 

 I advanced was the possible importance of this continued influence on the 

 retained egg and embryo as giving the clue to its different development as 

 compared with the free spore. The idea of the retention of the egg may be 

 made a little clearer by distinguishing between effective and ineffective 

 retention. The mere fact that an egg develops within an oogonium or arche- 

 gonium only amounts to ineffective retention if mutual relations are not 

 established between it and the gametophyte. Thus retention is ineffective in 

 Vaucheria or (Edorjonhnn, but it is not clear that it is so in C'ohoc/iccte. As 

 regards the egg in the archegonium, however, there is evidence of mutual rela- 

 tions between the embryo and the prothallus. We see changes in the calyptra 

 and in other parts of the prothallus following on the presence of a developing 

 embryo. On the other hand, the symmetry of the normal embryo and the 

 position of its primarj' members is determined by the relation to the prothallus, 

 and not by the external influences of light, gravity, &c. 



In trying to form any idea of the nature of the influences which result in the 

 cyclical correlation between gametophyte and sporophyte in the normal life- 

 liistory the deviations from the normal met witli in apogamy and apospory 

 are of assistance ; they have an importance for causal morphology which 

 was almost wholly overlooked when the problem of alternation was stated 

 from the phyletic point of view. The first examples of apogamy and apospory 

 described in the ferns were associated respectively with an absence of the sexual 

 organs or of sexual reproduction and imperfection or absence of the sporangia. 

 Later work has not diminished the possible importance of this sexual arrest or 

 soral arrest in some cases, but has shown that the problem is by no means 

 simple or direct. To properly discuss it would require the consideration of 

 examples, not only of the transitions between plant and prothallus, but of the 

 mix-up of tissues and members of the two generations. Our knowledge of these 

 has increased greatly, but in most cases we are only acquainted with the 

 phenomena without being in a position to understand them. Let me remind yon 

 of some types of apogamy : apogamous development of an ovum ; direct 

 apogamy as a constant character of the particular species or variety ; direct 

 apogamy with the occurrence of structures intermediate between gametophyte 

 and sporophyte ; induced apogamy in potentially normal prothalli ; the develop- 

 ment of isolated leaves, roots, ramenta, or sporangia upon a potentially normal 

 prothallus. On the other hand, we have as manifestations of apospory : 

 apospory as a regular characteristic of certain varieties ; aposporous develop- 

 ment from the attached leaves of young plants ; induced apospory from the 

 primary leaves of normal ferns ; induced apospory by the arrest of spore pro- 

 duction ; and the occurrence of intermediate growths between sporophyte and 

 gametophyte. Clearly no one formula will cover all these cases. The nuclear 

 facts we now possess are of great interest, but do not give an explanation of the 

 phenomena. The whole subject is a most promising one for critical and 

 thorough experimental work, all that has yet been done in this direction being 

 of the nature of prospecting. 



Let us now assume that the normal alternation is due to cyclical correlation 

 and try to analyse this a little further in the light of the facts of apogamy and 

 apospory. The simplest cases of direct apogamy show a fern sporophyte con- 

 tinuous with the cushion of the protliallus, but with the relative position of its 

 members the same as that of a plant developed from a fertilised egg. We are 

 naturally inclined to think first of a change to the alternative sy-stem of rela- 

 tions and to remember that in the case of the normal embryo this takes place 

 in a similar relation to the prothallus. When, however, we consider the strange 

 cases of the perfect development of isolated members or tissues of the sporo- 

 phyte on or in the prothallus, we seem forced to think further of special 

 formative influences that are of the nature of substance rather than of a system 

 of relations. We are confirmed in this by the fact that a number of cells may 

 be simultaneously and collectively influenced, and that the influence may be 

 reversed. This holds both for apogamy and apospory ; in the latter when 

 the prothalli develop upon leaves attached to the plant, it is difficult to see 



