CHAPTER 6 

 FERTILIZATION 



In gymnosperms the pollen grains usually land directly on the 

 nucellus, while in angiosperms they are deposited on the stigma. 

 There are various agencies which serve to bring about this transfer 

 of pollen from the anthers to the stigma, but since this is primarily 

 an ecological subject and information on it is readily available else- 

 where, it need not be dealt with here. It is sufficient to say that, 

 in the condition in which they are discharged from the anther, the 

 pollen grains show considerable resistance to environmental changes. 

 Sometimes they retain their viability for several weeks, and with 

 proper methods of storage this period can be prolonged still further 

 (see Chap. 12). 



Germination of Pollen. Exact information on the time taken by 

 pollen to germinate on the stigma is available for only a few plants, 

 but the following examples will illustrate the range that has been 

 observed: 2 days in Garrya elliptica (Hallock, 1930), 3 hours in 

 Reseda spp. (Eigsti, 1937) ; 2 hours in Beta vulgaris (Artschwager and 

 Starrett, 1933); and 5 minutes in Taraxacum kok-saghys (Poddub- 

 naja-Arnoldi and Dianowa, 1934), Zea mays (Randolph, 1936), 

 and Hordeum distichon (Pope, 1937). In Saccharum oflicinarum 

 (Artschwager et al, 1929) and Sorghum vulgare (Artschwager and 

 McGuire, 1949) germination takes place almost immediately. 



The first step in germination is the expansion of the pollen grain 

 by the absorption of liquid from the moist surface of the stigma and 

 the protrusion of the intine through a germ pore. The small tubular 

 structure which arises in this way then continues to elongate, mak- 

 ing its way down the tissues of the stigma and style. Only the 

 distal part of the tube has living cytoplasm, and as the nuclei pass 

 forward callose plugs are left in the empty portions behind them. 



Most pollen grains are monosiphonous, i.e., only a single pollen 

 tube emerges from each pollen grain; others, like those of the Mal- 

 vaceae, Cucurbitaceae and Campanulaceae, are polysiphonous. 

 In Althaea rosea 10 tubes, and in Malva neglecta even 14 tubes, are 



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