Instant Notes: Plant Biology

with sporangia at their tips (Fig. 3). Their relationships are obscure, perhaps

with the pteridosperms, but their leaves resemble microphylls such as possibly

those of progymnosperms (Topic Q4). If they are related to the progymno-

sperms it suggests that Cordaitales and conifers may have developed the seed

habit independently from pteridosperms and other groups.

There were several other groups of seed plants found only as fossils that had

died out by the end of the Cretaceous period. Five groups of seed plants

survive. Of these, the conifers have a fossil history extending right back to the

Carboniferous, the cycads to the Triassic and the Ginkgoales as far as the

Permian period. The Gnetales and the angiosperms are more recent, probably

appearing first in the Jurassic period.

R1 – Early seed plants 295

Microsporangia

1 mm

Fig. 3. Cone-like male reproductive structure of the Cordaitales.