Allgemeines 293 



spore-bearing plants as are heterosporoiis, difficulties in the 

 way of successful fertilisation exist, it being necessary that 

 both kinds of spores shall germinate together, as vveli as in the 

 presence of an adequate water supply. It is pointed out that 

 in the case of the great cryptogamic trees of the Palseozoic 

 period, this difficulty must have been serious, and it was per- 

 haps due to this that the series of adaptations leading up to 

 seed formation owed their first inception. The first step would 

 be the bringing together of the two kinds of spore on the 

 parent plant, and this adaptation is one of the constant charac- 

 teristics of the seed-bearing plants. 



It is shown how the Cycadean method of fertilisation holds 

 exactly the middle place between the purely cryptogamic pro- 

 cess, where the active male cells accomplish the whole journey 

 to the egg by their own exertions, and the method typical of 

 seed-plants, where these cells are little more than mere pas- 

 sengers carried along by the grov/th of the pollen-tube. There 

 are three chief adaptations in the Cycads in favour of polli- 

 nation: the envelope of the seed with its narrow opening down 

 which the pollen grains are guided; the pollen-chamber below, 

 in which they are received; and the pollen tube. The ovule of 

 a Cycad differs however from the spore-sac of a Cryptogam, 

 not only in the solitary megaspore — a condition already 

 reached among the Water-ferns — but in being firmly imbedded 

 in the surrounding tissue, and in remaining throughout as an 

 integral part of the ovule. On the other band it is pointed out 

 that in the Cycads and also Gingko, the seed does not con- 

 "tain an embryo, and that the ripening of the seed itself is not 

 dependent on the development of an embryo as is the case in 

 the higher Phanerogams. Among Palseozoic seeds also there 

 is no case knov/n in which the seed contains an embryo, and 

 thus there are strictly speaking no „seeds" of Palaeozoic age 

 according to current clefinitions. Hence it seems not improbable 

 that the development of an embryo in the ripening seed was a 

 later device, that in the older seed plants the period of rest 

 came immediately after fertilisation, and that the growth of the 

 embryo, when once started, went on rapidly and continuously 

 to germination, in which case a seed with a recognisable embryo 

 would rarely be preserved. 



In considering the historical question, from what group of 

 spore-bearing plants were seed-plants derived, one thing is 

 piain. The stage of heterospory was the immediate precursor 

 of seed-formation. Among the Higher Cryptogams possessing 

 heterospory there is only one line — the Ferns — which has 

 yielded truly intermediate types between the two great groups 

 Ol Spore-bearers and Seed-bearers. It is pointed out that the 

 Cycads, not only by certain of their recent characters, but by 

 those of their Mesozoic ancestors, show some indications of 

 such an intermediate position. The Cycadofilices of Upper 

 Palaeozoic times clearly combine Cycadean with Fern charac- 



