THE AXGIOSPERMAE 1433 



structures in Angiosperms, first within the carpel and secondlv within the 

 ovule, contact between the gamete nuclei can only be established by means 

 of that extraordinary structure, the pollen tube, which provides both a 

 channel of entry and a protection for the male gamete from external dangers. 

 Even the few peculiar instances where pollen grains gain access to the ovary 

 through an open stylar canal or the nucellar pollination of Rheum australe 

 (see p. 1284) are no exception, since the nucellus or the micropvle remains 

 to be traversed by a pollen tube, as in Gymnosperms. 



We have previously traced the development of the microspore or pollen 

 grain up to the point of its liberation from the anther and we have dealt in 

 Chapter XXI \' with the adventures of its transference to the stigma. 



Germination of the Pollen Grain 



When liberated from the anther the grain may contain either two cells 

 or three, that is to say that the generative cell may have already divided or it 

 may not. The difference is not accidental, for it is constant for species and 

 indeed often for whole families. Schiirhoff^ has suggested that the two- 

 nucleate condition is more primitive than the three-nucleate and that the 

 latter condition prevails in the more advanced families. There is not enough 

 information available to establish this conclusion, which obviously is phylo- 

 genetically important. Some at least of the pollen grains reported as two- 

 nucleate were immature and we know from experiment that the balance 

 between the conditions may be upset by artificial nutrition and other factors 

 which may induce both types of grain in the same plant. Moreover in some 

 two-nucleate grains the generative cell is already in mitotic prophase when 

 the grain is shed. A survey of the records shows a rather irregular distri- 

 bution of the two conditions among the families, so that the known facts 

 scarcely seem to bear the weight that Schiirhoff has placed on them, though 

 the question deserves further investigation. 



The content of the pollen grain is a true gametophyte, as is evidenced by 

 the finding of both parental types of pollen grain in the anthers of the F^ 

 generation of certain hybrids. In other words the building of the grain is 

 internally controlled by the haploid genotype. What comparisons we may 

 draw with the male gametophytes of other phyla is, however, an open ques- 

 tion. Reduction has gone too far to allow of more than a guess that the game- 

 tophyte in the pollen grain may represent a single antheridium. Otherwise 

 an agnostic attitude is advisable. 



The longevity of pollen or the time during which it retains its ability to 

 germinate is a matter of importance to the plant breeder. It depends not only 

 on the plant but on the prevailing conditions, of which humiditv is the most 

 important. Only a few species survive long in air of relative humidity 

 60 to 90 per cent. [Hippiiris is an example) and most are longer lived in air of 

 30 per cent, humidity or less; many survive longest when desiccated over a 

 drying agent. Some pollen, like that of Hippiiris, is insensitive to changes of 

 humidity, but most species have a definite pptimum. Wetting followed by 



