12 Bulletin of the University of Texas 



species to perpetuate itself and to become distributed over the 

 earth. 



In the typical, complete flower, four sets of organs are pres- 

 ent: the calyx, the corolla, the stamens, and the pistil. The 

 calyx is the outer whorl of leaves, usually greenish in color, which, 

 before the flower opens, functions in much the same manner as 

 the bud scales, preventing the delicate inner parts from suffering 

 injury due to water loss. The calyx is composed of several sepa- 

 rate leaves known as sepals. The corolla is the showy colored part 

 of most flowers, and is made up of a number of colored leaves, 

 petals, which may be entirely separated from each other, or in 

 some cases are more or less united at the base. The stamens, 

 .several in number, are usually separate, filamentous structures 

 which bear an enlarged body at the upper end known as an 

 anther or pollen case. Their function is to produce the pollen. 

 The pistil consists of three parts : the ovary or basal enlarged por- 

 tion which is usually somewhat inflated and bears the ovules or 

 young seeds; the style, a filament about the size of the stamens 

 and growing up to about the same level with their tops, or some- 

 times projecting beyond them, and the stigma, or the swollen 

 expanded end of the style. 



The stamens and the pistil are the essential parts of the flower, 

 while the calyx and corolla are known as the accessory parts. 



When the flowers are fully developed the anther case breaks 

 -open and the pollen is shed from it. In order for a seed to be 

 formed, some of this pollen must fall upon the stigma and de- 

 velop into a long burrowing tube which grows down through the 

 style into the ovule or young seed. This is known as the pollen 

 tube, and each ovule must receive such a tube or die. The pollen 

 tube is microscopic in size and bears the male reproductive, or 

 .sperm cells. Inside the young seed or ovule, the female repro- 

 ductive cell, the egg cell, is borne. The pollen tube penetrates 

 the opening to the ovule and delivers the male cell to the female. 

 'These two cells then fuse completely into one. This process is 

 known as fertilization, and the embryo plant develops from the 

 fertilized egg cell. The embryo plant with the tissue surrounding 

 it is called the seed. 



Of all the processes carried on by plants, the method by 

 which a seed is formed is perhaps the most striking and interest- 



