CH. XIII] 



THE CONIFERS 



519 



a short pollen tube, by opening of which are set free two 

 very large multiciliate sperm cells. These swim freely 

 in the liquid of the pollen chamber and the now open 

 end of nucellus and embryo sac, pass down between the 

 neck cells of the archegonia, and fertilize the large egg cells. 

 This persistence of the free-swimming sperm cells in the 

 Spermatophytes, where it is confined to the Cycads and to 

 Ginkgo (mentioned below), is a very striking fact, of great 

 interest in connection with 

 plant evolution. 



Other forms of Cycads bear 

 the ovules in cones, as in 

 Zamia, common in Florida, 

 and typical of the many with 

 short, tuberous, rough stems. 

 Great numbers of fossil Cy- 

 cads are known, especially in 

 the Jurassic Period, and our 

 existent forms are a decadent 

 remnant of an extensive 

 ancient Cycad flora. Their 

 characters link them very perfectly with the extinct 

 Cycadofilicales, a group intermediate between Cycads and 

 Ferns, and thence with the ancient Ferns (Fig. 275) . 



Related to the Cycads, but commonly placed in a separate 

 order, is the Ginkgo, or Maiden Hair Tree of Japan, often 

 grown in America. It is dioecious, and has free-swimming 

 sperm cells, but the solitary ovules ripen to fleshy drupe- 

 like fruits (Fig. 365). 



ORDER 2. CONIFERALES : THE CONIFERS. These are the 

 most abundant and familiar of existent Gymnosperms, for 

 they include the Pine (Pinus), Fir (Abies), Spruce (Picea), 

 Hemlock (Tsuga), Arbor Vitse (Thuja), Cedar (Chamcecy- 

 paris), Cypress (Cupressus), Redwood (Sequoia), and Larch 

 (Larix), having cones, with Juniper (Juniperus) and Yew 



FIG. 365. Ginkgo biloba; X 

 Leaf and unripe fruits, one sectioned. 

 (From Kerner.) 



