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A TEXTBOOK OF THEORETICAL BOTANY 



which was innermo&t in the tetrad stage. The spindle is short and asym- 

 metrical, being shorter and more pointed on the side of the generative 

 nucleus, which leads to differences in the arrangement of the chromosomes 



Fig. 1281. — Tradescantia bracteata. Successive stages in the 

 first mitosis in the pollen grain. The density of stippling 

 represents the distribution of protein and ribonucleic 

 acid in the cytoplasm and of desoxyribonucleic acid in 

 the nuclei. There is a high concentration of the latter in 

 the generative nucleus as compared with the vegetative 

 nucleus. {After La Coiir.) 



at the two poles. The nuclear division is strictly equational, nevertheless 

 the two nuclei formed are soon differentiated, one becoming the generative 

 nucleus, from which the two male gametes will be formed, the other be- 

 coming the vegetative nucleus of the pollen grain, and later the "tube 

 nucleus". 



After mitosis the vegetative nucleus expands and loses much of its 

 staining capacity. The generative nucleus, on the other hand, stains densely 

 and is rich in desoxyribosenucleic acid (D.N. A.). In contrary fashion, the 

 cytoplasm on the side of the vegetative cell gains in staining capacity and is 

 rich in protein and in ribonucleic acid (R.N. A.). The cytoplasm surround- 

 ing the generative nucleus loses its staining capacity, having very little pro- 

 tein and no R.N. A. It would seem that this special cytoplasm is derived 

 directly from the nuclear sap of the parent nucleus and is not invaded by 

 R.N. A. from the general cytoplasm. These differences have been empha- 

 sized by La Cour as the key to the nuclear differentiation. The vegetative 

 nucleus is supplied with materials which are factors for growth; the genera- 

 tive nucleus lies in a medium associated with mitosis, leading to its further 

 division. 



A delicate cell- wall is formed between the two nuclei, which is curved 



