CLASSIFICATION OF PLANTS. 7 



2. Angiosperms, which produce true fruits enclosing 

 the seeds. In this group there are two well-marked sub- 

 divisions differing in the structure of the seed and stem. 

 They are the Endogens, or inside growers, with seeds hav- 

 ing one seed-leaf only, as the grasses and the palms ; and 

 the Exogens, having outside-growing woody stems, and 

 seeds with two seed-leaves. Most of the ordinary forest- 

 trees of temperate climates belong to this group. 



On referring to the geological table, it will be seen 

 that there is a certain rough correspondence between the 

 order of rank of plants and the order of their appearance 

 in time. The oldest plants that we certainly know are 

 Algaa, and with these there are plants apparently with 

 the structures of Thallophytes but the habit of trees, and 

 which, for want of a better name, I may call Protogens. 

 Plants akin to the Rhizocarps also appear very early. 

 Next in order we find forests in which gigantic Ferns and 

 Lycopods and Mare's-tails predominate, and are associated 

 with pines. Succeeding these we have a reign of Gym- 

 nosperms, and in the later formations we find the higher 

 Phsenogams dominant. Thus there is an advance in 

 elevation and complexity along with the advance in 

 geological time, but connected with the remarkable fact 

 that in earlier times low groups attain to an elevation 

 unexampled in later times, when their places are occu- 

 pied with plants of higher type. 



It is this historical development that we have to trace 

 in the following pages, and it will be the most simple 

 and at the same time the most instructive method to 

 consider it in the order of time. 



