CHAPTER XXI 



FERTILIZATION AND POLLINATION 



276. Fertilization. — Seeds result from the union of two 

 elements or parts. One of these elements, a nucleus of a 

 plant-cell, is borne in the germinating pollen-grain. The 

 other element, an egg-cell, is borne in the ovary. The pollen- 

 grain falls on the stigma. (Fig. 218.) It absorbs water or the 

 juices exuded by the stigma and grows by sending out a 

 tube. (Fig. 219.) This tube grows downward through the 

 style, absorbing food as it goes, and finally reaches the egg- 

 cell in the interior of an ovule in 

 the ovary, and fertilization by the 

 union of the two nuclei takes place. 

 The ovule then develops into a 

 seed. The growth of the pollen- 

 tube is often spoken of as germi- 

 nation of the pollen, but it is 

 not germination in the sense in 

 which the word is used when 

 speaking of seeds. 



277. In order that 

 the pollen may grow, 



the stigma must be ripe. At this stage, the 

 stigma is usually moist and sometimes sticky. 

 The pollen is held by the mucilaginous secre- 

 tion on the stigma. The stigma may be barbed 

 or feathery and hold the pollen by this means. 

 Observe the stigma of some of the lilies. In 

 corn the "silk" constitutes the style, and the 

 stigma is feathery. A ripe stigma is said to be 

 (133) 



218. B. pollen of plum escaping 

 from anther. A, pollen germin- 

 ating on the stigma. Enlarged. 



219. 

 Pollen - grain 

 germinating. 

 Magnified. 



