1434 A TEXTBOOK OF THEORETICAL BOTANY 



drying is very lethal. We may take a period of 30 to 40 days as an average 

 for the survival of pollen in air, but there are some striking exceptions. 



The longest-lived pollen recorded is that of Pinus sylvestris, which can 

 survive for 279 days over sulphuric acid, but only about 70 days in free air. 

 Holman records a small germination of Typho pollen after 336 days (condi- 

 tions not stated) but only 56 per cent, germinated after 158 days. Other 

 long-lived pollens are those of Viola odorata (235 days dry, but only 35 days 

 in air). Primula elatiur (179 days dry, 56 in air), and Priimis padiis (181 days 

 dry, 15 in air). The shortest lived is that of Secale cereale, which does not 

 survive more than 12 hours under any conditions. Most other Gramineae 

 have short-lived pollen. Longevity seems to be hereditarily determined 

 rather than ecologically. Most spring flowering plants have pollen which 

 is relatively insensitive and is long lived, though this is not an ecologically 

 important feature in their case. 



Pollen germination occurs under a much wider range of conditions than 

 those suitable for the continued growth of the pollen tube. Pollen is very 

 sensitive to moisture, which it rapidly absorbs, with swelling of the grain, 

 but in pure water germination has scarcely time to begin before the grains 

 burst. Tradescantia and Impatiens pollens are exceptional in their very 

 rapid germination in pure water without bursting. Indeed 10 per cent, 

 sucrose is enough to inhibit the germination of Tradescantia. 



The fact that artificial germination of pollen is best carried out in sugar 

 solutions, often of surprisingly high concentration, up to 60 per cent, in some 

 cases, has given rise to the belief that the sugars secreted by the stigma have a 

 nutritional value in promoting the germination of pollen. In most cases this 

 does not seem to be true. The nature of the sugar does not seem to be of so 

 much importance as its concentration and the conclusion is that its principal 

 role is the osmotic control of swelling. There are a few exceptions where 

 special sugars may be needed for heterotrophic nutrition. For example, the 

 pollen of Mussaenda requires traces of fructose and that of Cerastium and 

 some other Caryophyllaceae requires 20 to 30 per cent, of lactose for experi- 

 mental germination. Dilute acids generally promote germination, the pollen 

 of Ericaceae being especially stimulated by dilute malic acid. 



When the grain has germinated the pollen tube penetrates the stigma, 

 usually between the cells, but where there is an open stylar canal most of 

 the tubes pass down it, keeping contact with its walls. The action of penetra- 

 tion does not seem to be chemotropically conditioned, under ordinary cir- 

 cumstances, but is primarily hydrotropic aided by negative aerotropism. 



The stigmatic surface is generally papillose and the pollen grains are 

 held between the papillae and germinate there. Germination usually occurs 

 with extraordinary rapidity, in a few minutes in most cases. Jost records 

 that five minutes after pollination, pollen tubes had already penetrated the 

 stigmatic branches in Secale. The cells of the stigma have the appearance 

 of secretory cells, dense cytoplasm and large nuclei, but they seem to be 

 very loosely joined and are easily separated by maceration, even in water. 

 It is between them, not into them, that the pollen tube penetrates (Fig. 



