CHAPTER XIX. 

 FERTILIZATION AND DEVELOPMENT OF THE SEED.* 



299. Fertilization consists in the union of two cells, or of the 

 nuclei of two cells of a different nature, a sperm or male nucleus 

 with an egg or female nucleus. The result is a stimulus or impulse 

 given to the fertilized egg to develop and form an 

 embryo plant which later can develop into a plant 

 like the parent. The male, nucleus, or sperm 

 nucleus, is derived from the pollen grain" which 

 is formed in the anther of the stamen. The egg 

 Fig. 141. is developed in the embryo sac within the ovule, 

 poii a grainftrii e which is in turn formed in the ovary, a part of 



Hum. The smaller , i ,-\ 



cell is the genera- the pistil. 



300. The formation of the pollen, the sperm 

 cells, and pollen tube. The pollen is formed, as we have seen, in 

 the anther. When the pollen grain is very young it consists of a 

 free cell in the pollen sac. The cell wall encloses the protoplasm, 

 the living substance, and within this is a very important organ of the 

 cell, the nucleus, w r hich is a more or less rounded body, finely granu- 

 lar and usually appearing denser than the protoplasm. As the pol- 

 len grain ripens a change takes place in its contents as follows. The 

 nucleus divides into two nuclei, and very often a, curved thin cell 

 wall is formed separating a small mass of the protoplasm with one 

 nucleus from the larger mass containing the other nucleus, as shown 



* The study of the processes of fertilization and the development of the seed 

 requires special preparation of material and the use of technical methods 

 which could not be employed by students in the first-year course. It is 

 desirable, however, that students should have a general knowledge of these 

 processes. This chapter is presented for this purpose. It can be supple- 

 mented, if desirable, by demonstrations of microscopic material prepared 

 by the teacher or purchased for the purpose. 



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