172 R- VENDRELY 



b. Pollen Formation 



The problem of the formation of the pollen grain is not quite clear at 

 the present time. It has been studied photometrically by Schrader and 

 Leuchtenberger/* Bryan,*" and Pasteels and Lison*^ on Tradescantia and 

 by chemical methods by Ogur et al}^ on Liliuni longiflorum. All these 

 authors have pointed out that the late microspore contains an amount of 

 DNA much higher than the expected value for a haploid cell. Yet, just 

 before prophase, in the microsporocyte the DNA content of the nucleus 

 represents four times the haploid value. This amount of DNA is exactly 

 shared in the four following microspores, but there is a postmeiotic increase 

 in this DNA,*"' *^ so that the microspore nucleus finally contains an amount 

 of DNA of the same order of magnitude as does the diploid nucleus of the 

 root tip or of the bud scale epidermis.^*' ^°' ^^ The microspore mitosis occurs 

 at this stage, and in the two nuclei thus formed in the pollen grain — the 

 vegetative nucleus and the germinative nucleus — there is a further gradual 

 increase in the DNA content. According to Pasteels and Lison^* this post- 

 mitotic increase of the DNA occurs only in the generative cell nucleus which 

 contains the diploid value. Bryan*" could not measure the DNA in the dif- 

 ferentiating generative nucleus, but he described an increase of the DNA 

 in the vegative nucleus. Ogur et alP by gross chemical analysis, have shown 

 that at anthesis the pollen grain contains a very high value of DNA (about 

 eight times the haploid value). 



It is difficult to draw any conclusion from these few investigations on the 

 formation of the pollen grain, but it is possible that the biochemical process 

 involved here is somewhat different from that of spermatogenesis in animals. 

 On the whole, the general relation between the number of chromosomes and 

 the DNA content of the nucleus is not always so exact in plant as in animal 

 nuclei. The interpretation of this phenomenon (polyteny , or other processes) 

 is still under consideration. 



c. DNA and Metabolism 



The possibility of extrusion of DNA (Feulgen-positive material) out of 

 the nucleus into the cytoplasm has been described in some cases. Lison and 

 Fautrez-Firlefyn"' *^ have reported in certain oocytes of a crustacean, Arte- 

 mia salina, an appreciable increase in the DNA of the nucleus which they 

 think represents a considerable degree of polyploidy. It is followed by an 

 extrusion of the DNA out of the nucleus and the death of the cell. Accord- 

 ing to these authors, the exaggeration of the process of polyploidy (to 

 eight times and more the haploid value) would be a lethal phenomenon for 

 cell. On the other hand, Schrader and Leuchtenberger,*^ working on the end 



85 J. Pasteels and L. Lison, Compt. rend. 233, 196 (1951). 



8« L. Lison and N. Fautrez-Firlefyn, Nature 166, 610 (1950). 



" F. Schrader and C. Leuchtenberger, Exptl. Cell Research 3, 136 (1952). 



