226 EVOLUTION OF PLANTS 
back to the Devonian, thus being one of the oldest 
genera known in the whole class. During the Carbo- 
niferous there appeared numerous arborescent forms, 
the principal genera being Lepidodendron and Sigil- 
laria. These trees exhibited a secondary growth of 
the stem, like that of Conifers, and the appearance 
of these was probably not unlike that of the modern 
coniferous trees, suggesting that the latter may be 
really related to forms like Lepidodendron. These were 
heterosporous like Selaginella, but apparently hetero- 
spory was not so pronounced. 
Both of the lowest existing types of seed-bearing 
plants, the Cycads and Gingko, are found fossil. They 
probably originated in the later Paleozoic formations, 
perhaps in the later Carboniferous. After the Carbo- 
niferous they increase rapidly in numbers, reaching 
their maximum in the Mesozoic, after which they 
diminish in numbers, giving way before the more 
specialized Conifers and Angiosperms. Many of the 
fossil cycadean remains are very complete, and often 
are obviously much like the existing genera, several of 
which, including the genus Cycas, undoubtedly occur 
fossil. 
The peculiar genus Gingko, now reduced to a single 
species, is represented by numerous fossil species, the 
oldest dating back to the Permian. 
The Cordaitezs comprise a group of entirely extinct 
plants which have been considered to be, in many ways, 
intermediate between the Cycads and Conifers. They 
are most abundant in the coal measures, but occur 
as early as the Devonian. They have been so perfectly 
preserved, in some instances, that the structure of the 
