534 BOTANY 



The earliest Lycopods occur in the Devonian formations, and the 

 group reached its greatest development in the Carboniferous. The 

 arborescent Lepidodendrons and Sigillarias became extinct toward 

 the end of the Palaeozoic era, and the later forms are all smaller 

 and more like the existing species. 



Gymnosperms 



The Gymnosperms, as might be expected, are found before any 

 evidence of the existence of Angiosperms is apparent. The earliest 

 remains of gymnospermous affinities occur in the Devonian, but 

 these belong to a peculiar class, the Cordaiteae, which became extinct 

 toward the end of the Carboniferous. These were trees with large 

 leaves, recalling those of Monocotyledons, but the structure of the 

 stem was, on the whole, much like that of the Coniferae. The flowers 

 and seeds have been preserved in some cases in a remarkable state 

 of perfection, so that the structure has been made out in detail. 

 It is evident that the gametophyte, especially the male gametophyte, 

 was better developed than in any living Spermatophytes. Within 

 the pollen-spore a cellular body can be seen which is, perhaps, com- 

 parable to the group of sperm-cells within the antheridiuin of the 

 heterosporous Pteridophytes. So perfectly are the ovules preserved 

 in some instances, that the archegonia can be recognized, and the 

 pollen-grains lying in a pollen chamber much like that found in the 

 ovule of the Cycads. It has been suggested that the Cordaiteae 

 were synthetic forms, uniting the characters of Cycads and Conifers, 

 although their affinities, on the whole, are rather with the latter. 



Ginkgoales. Of the existing Gymnosperms, the oldest type is 

 Ginkgo, now reduced to a single species, G. biloba, of China and 

 Japan. This genus can be traced back to the early Mesozoic forma- 

 tions, and it may possibly have existed in the later Palaeozoic, where 

 fossils evidently related to Ginkgo certainly occur. It seems possi- 

 ble, however, that the Palaeozoic forms were more nearly related to 

 the Cordaiteae, which show affinities with Ginkgo. 



Cycadales. The Cycads, like Ginkgo, represent a very ancient 

 type of Spermatophytes, and are also met with abundantly in a 

 fossil state. They reached their greatest development during the 

 Mesozoic, but there are Palaeozoic fossils of undoubted cycadean 

 affinities, although it is doubtful, in most cases, whether they are 

 true Cycads. Among the most important of recent discoveries is 

 the fact that many Palaeozoic fossils, supposed to be Ferns, are 

 really forms intermediate between the Ferns and Cycads, and 

 included by some botanists in a special class, the Cycado-filices. 

 Of these may be mentioned the genera Heterangium, Lyginodendron. 

 and Medullosa. Of the various types of fossil Cycads, some are not 



