162 EVOLUTION OF PLANTS 



THE GYMNOSPERM.E 



In the Gymnosperms the flowers are of the simplest 

 character, consisting entirely of sporophylls of one 

 kind. Macrospores and microspores are always borne 

 in different flowers and very often upon different plants. 



THE CYCADS (Cycadacece) 



Without question the lowest types of seed-bearing 

 plants known are the Cycadacese, a group of palm-like 

 plants of which the best known is the so-called " sago- 

 palm" of the greenhouses, Cycas revoluta. About sev- 

 enty-five living species of Cycads are known, widely dis- 

 tributed through the warmer regions of both the Old and 

 the New worlds. Most of them are strictly tropical, but 

 one species, Zamia integrifolia, is found as far north as 

 Florida, and Cycas revoluta probably extends beyond the 

 northern tropic in Japan. They recall in many ways 

 certain ferns, and a careful examination of the tissues 

 of the sporophyte shows that these resemblances are 

 more than superficial. The tissues of the fern-like 

 leaves resemble those of the lower ferns, and the leaves 

 when young are coiled up much as in the ordinary ferns 

 (Fig. 40, F). The plant, however, may develop a 

 primary tap-root like that of the Conifers or Dicotyle- 

 dons, and there is a more or less marked secondary 

 thickening of the vascular bundles of the stem, which, 

 however, also occurs in a few ferns. 



In Cycas the macrosporangia are borne upon leaves 

 which differ but slightly from the ordinary ones (Fig. 

 40, A). The sporangia are very large, sometimes being 



