1903.] on the Origin of Seed-hearing Plants. 341 



One thing is plain : the stage of heterospory was the immediate 

 precursor of seed-formation, and it was from some group of Crypto- 

 gams producing spores of two kinds, that the Seed-plants sprang. 

 Such heterosporous groups are however known in three of the main 

 phyla of the higher Cryptogams. 



In the Lycopod series, we have, among their living rei^resentatives, 

 pronounced heterospory in Selnginella and Isoetes ; among the Palaeo- 

 zoic Lycopods it was commoner still. Within the class of the Ferns 

 we have the heterosporous Water-ferus ; in the third series, that of 

 the Horsetails, we have, it is true, only homosporous forms now living, 

 but in Palaeozoic times a well-marked differentiation of micro- and 

 megaspores was attained, though less extreme than in the other 

 two lines. 



So far, therefore, there is no reason why the early Seed-plants 

 might not have had family relations with any of the great pterido- 

 phytic phyla, and as a matter of fact, all three lines have been 

 championed by one botanist or another as the probable ancestors 

 of the Seed-plants. 



The Horsetail stock, though it attained an extraordinary de- 

 velopment, shows no further sign of transition towards the higher 

 plants. 



The case for the Lycopods is stronger, and indeed they were 

 long the " favourites " and were commonly regarded as lying nearest 

 the true line of spermophytic descent. This idea was specially based 

 on the mode of development of the spore-sacs, which has much in 

 common with that of the pollen-sacs and ovules of Phanerogams, and 

 this, combined with the occurrence of well-marked heterospory in 

 some genera, appeared to point to a relationship. But the former 

 character (the development of the spore-sac from a group of cells 

 instead of from a single one) is now known to be common to certain 

 Ferns, and to just those Ferns (the Marattiaceee, etc.) which prove to 

 be the most ancient, so that this argument has lost its weight. It 

 has lately been found, indeed, that some of the Carboniferous Lyco- 

 pods produced seed-like organs, presenting the most striking analogies 

 with true seeds, but the plants which bore them were in all other 

 respects Lycopods pure and simple, and the case appears to have 

 been one of homoplastic modification. There is no indication, as 

 yet, of any forms really transitional between the Lycopods and the 

 Spermophyta. 



The one line, which, so far, has yielded truly intermediate types, 

 is that of the Ferns. 



Among recent plants the Cycads, as we have seen, offer some 

 points of agreement with Ferns, sufficient to have led certain dis- 

 guished botanists, for example Sachs and Warming, to strongly 

 maintain their Fern-ancestry. The chief points of agreement 

 are : — 



1. The Fern-like foliage in some Cycads, and in many the mode 

 of folding of the leaflets in the bud. 



