FURTHER STUDIES ON GYMXOSPERMS. 



217 



rig. 284. 



Fertilization 



cas, sma. 



11 



group, that the sperm cells are well-formed spermatozoids. In zamia each 

 one is shaped somewhat like the half of a biconvex lens, and around the con- 

 vex surface are several coils of cilia. After the 

 pollen tube has grown down through the nucellus, 

 and has reached a depression at the end of the 

 prothallium (endosperm) where the archegonia 

 are formed, the spermatozoids are set free from 

 the pollen tube, swim around in a liquid in this 

 depression, and later fuse with the egg. In 

 gingko and cycas these spermatozoids were first 

 discovered by Ikeno and Hirase in Japan, and 

 later in zamia by Webber in this country. In 

 figs. 283-286 the details of the male prothallia 

 and of fertilization are shown. 



435. The sporophyte in the gymnosperms. — 



In the pollen grains of the gymnosperms we 



easily recognize the characters belonging to the 



spores in the ferns and their allies, as well as in 



the liverworts and mosses. They belong to the fertilization in cy 



J & spermatozoid fusing with the 



same series of organs, are borne on the same larger female nucleus of the egg. 



r , , , . The egg protoplasm fills the 



phase or generation of the plant, and are practi- archegonium. (From drawings 



cally formed in the same general way, the ^ Hirase and Ikeno -> 



variations between the different groups not being greater than those within 



a single group, These spores we have recognized as being the product of 



We are able then to identify the sporophyte as that phase or 



generation of the plant formed from the fertilized 



egg and bearing ultimately the spores. We see 



from this thai the sporophyte in the gyrnnosperms 



is the prominent part of the plant, just as we 



found it to be in the ferns. The pine tree, then, 



as well as the gingko, cycas, yew, hemlock- 



Spermatozoid of gingko, show- spruce, black spruce, the giant redwood of Cali- 

 ing cilia at one end and tail at . 



the other (After drawings by forma, etc., are sporophytes. 



Hirase and Ikeno.) W hile the sporan gj a (anther sacs) of the male 



flowers open and permit the spores (pollen) to be scattered, the sporangia of the 

 female flowers of the gymnosperms rarely open. The macrospore is developed 

 within sporangium (nucellus) to form the female prothallium (endosperm). 



436. The gametophyte has become dependent on the sporophyte. — In this 

 respect the gymnosperms differ widely from the pteridophytes, though we see 

 suggestions of this condition of things in isoetes and selaginella, where the 

 female prothallium is developed within the macrospore, and even in sela- 

 ginella begins, and nearly completes, its development while still in the spo- 

 rangium. 



the sporophyte. 



