Plant World Continued 



247 



In a short time the endosperm has surrounded the embryo sporophyte 

 and has filled in the growing ovule. This surrounding and nourishing 

 cell mass is now called the endosperm, which is neither gametophyte 

 nor sporophyte. 



As the ovule grows in size, its outer coat becomes thickened and 

 hardened, and the endosperm within enlarges and solidifies. A layer 

 of cells at the base of the ovule now becomes corky and checks the 

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

 seed. 



It will 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- 

 ber. 



The pollen grains produce one 

 of the sexual phases of the life his- 

 tory, the male gametophyte, which 

 forms the sperm nuclei ; the mega- 

 spore produces the other sexual 

 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." 



Fig. 151. 

 Median section of the flower of a Butter- 

 cup showing its constitutent parts. On the 

 outside (lowest down in the figure and shaded) 

 are the sepals of the calyx: within this the 

 large petals of the corolla of which three are 

 shown; within this and seated higher on the 

 axis are the numerous club-shaped stamens, 

 each of which bears four pollen-sacs. Cen- 

 trally in the flower are the numerous carpels, 

 one of which is dissected so as to show its 

 single ovule, or future seed. (From Bower 

 after Le Maout and Decaisne.) 



