TAXONOMY 



293 



periods than now. They differ from the Angiosperms in several 

 respects, viz.: they bear naked ovules on the edges or flat surfaces 

 of leaves called carpels, while Angiosperms bear covered ones; each 

 megaspore produces within itself a bulky prothallus, in the upper 

 portion of which originate one or more archegonia, while in Angio- 

 sperms no recognizable prothallus has been proven to exist; the 

 stored food tissue within their seeds is prothallial tissue loaded with 

 starch, etc., while that in Angiosperm seeds (endosperm) is developed 

 from the endosperm nucleus; the mode of growth of their stems is 

 always indefinite while that of Angiosperms is either indefinite or 

 definite. 



FIG. 160. Cycas revoluta, showing terminal bud of foliage-leaves just opening. 



(Gager.) 



The groups still extant are the Cycads or Fern Palms, the Gne- 

 tums, the Ephedras, the Ginkgos and the Conifers. Of these the 

 Conifers comprising over 300 species are the most numerous. Many 

 of them yield valuable products to pharmacy and the arts. 



The Conifers include the pines, spruces, hemlocks, cedars, firs, 

 arbor vitae, chamaecyparis, and larches. All of their number are 

 evergreen except the larches, which drop their foliage upon the 

 advent of winter. 



