SEED PLANTS 161 



may be compared directly with those of the Pterido- 

 phytes, which are sometimes grouped in a spike or 

 strobilus, as is seen in the horsetails and club-mosses, 

 and this strobilus is structurally much like the flower of 

 some of the lower Spermatophytes, especially the Conif- 

 erse. 



In the Cycads, which are the lowest known Spermato- 

 phytes, the foliar nature of the sporophylls is very 

 obvious (Fig. 40, A), but in the higher forms this is not 

 usually so evident, especially as regards the carpels. 

 In addition to the sporophylls, most of the higher Sper- 

 matophytes have accessory floral leaves, sepals and 

 petals, which, however, are by no means necessarily 

 present. 



The seed-bearing plants are commonly divided into 

 two great divisions, Gymnosperms and Angiosperms. 

 The former, which include the Cycads, the Conifers, and 

 a third less familiar order, the Gnetacese, or " joint-firs," 

 are characterized by having the macrosporangium borne 

 upon an open carpellary leaf; hence the name, Gymno- 

 spermse, or naked-seeded plants. In the Angiosperms, 

 the second group, the carpel (or carpels) forms a closed 

 cavity, the ovary, in which the ovules, and later the 

 seeds, are completely enclosed. It is this last group 

 which comprises the vast majority of the flowering 

 plants. 



