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- 
ere. 
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 Gnetacee, or “ joint-firs,” 
are characterized by having the macrosporangium borne 
upon an open carpellary leaf; hence the name, Gymno- 
sperm, 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. 
