52 THE ORIGIN OF THE SEED-PLANTS 
Seed-plants, and especially in the Gymnosperms. There the great dis- 
tinction is that, while the prothallus is comparable to that of a hetero- 
sporous Cryptogam, the megaspore (embryo-sac) remains permanently 
enclosed in the sporangium, which is itself enveloped in a highly orga- 
nised integument, becoming the seed-coat or testa. Pollination and (in 
most cases) fertilisation take place on the parent plant, and-the seed 
(testa, sporangium and prothallus) is, as a rule, only shed when the em- 
bryo has attained a certain development. It is unnecessary here to refer 
to the seed characters of the Angiosperms, which are altogether more 
advanced and more remote from the Cryptogamic phase. 
It has been generally assumed, since Hofmeister’s discoveries, that 
the Seed-plants were derived from heterosporous Vascular Crypto- 
gams, though the praticular group to which they traced their origin has 
been a subject of controversy. Although the homologies of the repro- 
ductive organs in Seed-plants and the higher Spore-plants can, to a 
considerable extent, be traced, there is nowhere any indication of a 
transition from one to the other, beyond the fact that in the Cycads 
and the Maidenhair tree, fertilisation is still effected by motile sper- 
matozoids, as in the Cryptogams. Yet they produce very highly deve- 
loped seeds. 
It thus came to be one of the chief problems of Palæobotany to seek, 
among the plants of past ages, some trace of a connection between the 
Spore-plants and the Seed-plants, so widely separated in the living Flo- 
ra. Of late years the tendency has been to find such a connection be- 
tween a certain group of Palæozoic Seed-plants and the Ferns. We will 
shortly explain how the present position was reached. 
Up to the year 1903 the current enumerations of Carboniferous land- _ 
plants showed that almost exactly half of the known species of that age 
were referred to the Ferns. The estimate was based almost wholly on 
the evidence of the fronds, which were so entirely fern-like that Sir: Jo- 
SEPH HOOKER. in 1848, spoke of Pecopteris as „the Fossil representati- 
ve, if not congener, of the modern Pteris.” and added, , 
bable that there are other genera of living ferns fossilised in the shales 
of the coal formation.” 1) 
It is true that as early as 1883, the Austrian palæobotanist Stur sta- 
It is not impro- 
1) J. D. Hooker, „On the Vegetation of the Carboniferous Period as compared 
with that of the present day,’ Memoirs of the Geological Survey of Great Britain, 
Volo Il, Patt. Il, p-401, 1848; 
