SECTION II 



THE INDIVIDUAL PLANT 

 CHAPTER I 



FERTILISATION, CYTOLOGY, AND EMBRYOLOGY 



The development of the sexual cells ^ in the cotton flower 

 bud is not marked by many special features, but the 

 intrinsic importance of the gametes is such that the main 

 characteristics of their microscopic history should be 

 examined, if only to facilitate their visualisation when 

 matters relating to heredity are under discussion. 



The life-story of the individual begins at the moment of 

 fusion between the male and female gametes, viz. , between 

 the generative nucleus of the pollen grain and the egg-cell 

 of the ovule. The unicellular zygote thus formed develops 

 by repeated division into an embryo enclosed in the seed- 

 coats, and the germination of this seed is the stage 

 commonly regarded as beginning the life-story. For 

 reasons both of precision and of ultimate convenience, we 

 shall commence the study of an individual plant at the 

 union of the gametes, first, however, describing the genesis 

 of the latter. 



The Gamete-mother Cells. — The young flower-bud 

 (Fig. l) develops centripetally from the primordia of 

 involucral bracts, calyx, corolla with staminal column, and 

 lastly ovary. The latter is composed of two to six carpels, 

 which originate as separate ring-primordia, developing 



