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 Palaeozoic 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 Cordaitese 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 



