246 GENERAL BIOLOGY 



the supply of water, so that the whole ovule becomes hardened to form 

 the seed. 



It \vill be noted, therefore, that the spermatophytes also show an 

 alternation of generations, the ordinary plant being- the sexless type. It 



is this ordinary flowering plant 

 which produces the microspores, or 

 pollen grains, and megaspores. In 

 the nuclear divisions which produce 

 these cells, the chromosome number 

 is reduced to half the original num- 

 Fig. i5i. ber. 



The pollen grains produce one 



ofThe" &Z wfthTn" tM f the S6XUal P ha S S f the life his- 



large petals of the corolla of which three are tory, the male gametophyte, which 



shown ; within this and seated higher on the J \ J 



axis are the numerous club-shaped stamens, forms the Sperm nuclei ', the mega- 



each of which bears four pollen-sacs. Cen- , , , 



trally in the flower are the numerous carpels, Spore prOdUCCS the Other SCX- 



one of which is dissected so as to show its -, _i__ 4.1,^ r^.^,^^1^. .^t, 



single ovule, or future seed. (From Bower Ual phase, the female gametophyte 



which bears an egg. Fertilization 



occurs by the fusion of a sperm cell with the egg; thus the nucleus of 

 the fertilized egg contains twice the number of the reduced amount of 

 chromosomes, one-half of which has been contributed by the sperm and 

 one-half by the egg. The fertilized egg develops into the embryo of the 

 seed which, upon germination, becomes the mature sporophyte or sexless 

 phase of the life history with its characteristic number of chromosomes. 



References : 



Strasburger, Noll, Schenck and Karsten, "A Textbook of Botany." 



Coulter and Chamberlain, "Textbook of Botany." Vols. I and II. 



Coulter, "Plant Structures." 



Wm. C. Stevens, "Plant Anatomy." 



C. W. Ballard, "The Elements of Vegetable Histology." 



C. S. Gager, "Fundamentals of Botany." 



Berger and Davis, "Principles of Botany." 



C. E. and E. A. Bessey, "Essentials of College Botany." 



Edson S. Bastin, "College Botany." 



Geo. Massee, "A Textbook of Fungi." 



F. L. Stevens, "The Fungi which Cause Plant Disease." 



Elizabeth M. Dunham, "How to Know the Mosses." 



Wm. N. Clute, "The Fern Allies." 



