280 University of California Puhlications in Botany [Vol. 5 



role of specific substances in determining whether or not pollen will 

 germinate. Latterly the views of ]\Iolisch and Burch emphasizing the 

 importance of specific substances are being replaced in the light of 

 results obtained by IMartin. Tokugawa and in the later work of Jost, 

 which indicate that general physical conditions are the determining 

 factors in pollen germination. Complete reference to and review of 

 the older and more recent literature is given by Martin (1913) and 

 Tokugawa (1914). Especially important is the question of water- 

 supply and its regulation. Jost has demonstrated in a rather wide 

 range of cases that restricted water-supply is the significant factor 

 in bringing about germination of pollen. ]\Iartin (loc. cif.) found that 

 the germination of the pollen of red clover depends upon a proper 

 water-supply; the range of variation in water-supply within which 

 pollen would germinate was limited; indeed, conditions giving free 

 water-supply to the stigma may lead to sterility. The results given 

 in table 2 above add fragmentary evidence along this same line. Thus, 

 the reaction of the pollen of N. Tah. var. nmcrophyUa in tap water 

 and in 10 per cent levulose is not susceptible of explanation on the 

 basis of osmotic pressure, but is intelligible with reference to the effect 

 of reagents upon the swelling of cell colloids. Inhibition of water 

 absorption could be brought about by the calcium salts of tap water, 

 a result in keeping with the general action of the alkali earths in such 

 connections (Cranner, 1914, p. 536). This general tendency to assign 

 greater importance to general physical conditions has, of course, been 

 increasingly apparent in the work being done on seed germination, in 

 which field the range of experiments has been far more extensive and 

 the results apparently conclusive. 



The complete failure of numerous efforts to bring about the 

 germination of the apparently good hybrid pollen made it seem best 

 to await the results of cytological studies on its condition before 

 attempting further germination experiments along the newer lines 

 indicated above. It now seems possible that the relatively few well- 

 rounded pollen-grains of the hybrid which appear to be normally rich 

 in c^vtoplasmic and nuclear substance are of abnormal constitution 

 with respect to chromatic content. In the preparations which we have 

 obtained ^showing the maturation divisions in the pollen mother-cells 

 of the Fj hybrid, the chromatin appears greatly fragmented or chromo- 

 some distribution is incomplete and chromosome fragments or lagging 

 chromosomes are seen in the equatorial zone or on the spindle in 

 telophase of the heterotypic division. It is possible that even with 



