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THE AMERICAN NATURALIST [Vol. XLIX 



the chances of combination between sperm and eggs we can interpret 

 the results. 



I make this quotation to show Morgan's viewpoint. It is 

 for him to say whether the following conclusions are ex- 

 tensions of his own or not. 



The tolerably constant rate of growth of pollen tubes in 

 the pistils of selfed flowers, compared with the great ac- 

 celeration of growth of the tubes from the pollen of other 

 plants as they penetrate nearer and nearer to the ovary, 

 undoubtedly shows the presence of stimulants of great 

 specificity akin to the "Individualstoffe" of Jost. We 

 are wholly ignorant of the nature of these stimulants, but 

 I am inclined towards a hypothesis differing somewhat 

 from his. Experiments by several botanists, which I 

 have been able partially to corroborate, point to a single 

 sugar, probably of the hexose group, as the direct stimu- 

 lant. The specific " Individuals t off e" I believe to reside 

 in the pollen grains and to be in the nature of enzymes of 

 slightly different character, all of which, except the one 

 produced by the plant itself for the use of its own pollen 

 or by other plants of identical germinal constitutions, 

 can call forth secretion of the sugar that gives the direct 

 stimulus. At least this idea links together logically the 

 fact of the single direct stimulus and the need of "Indi- 

 vidualstoffe " to account for the results of the crossing and 

 selfing experiments. But whether or not this be the cor- 

 rect physiological inference, the crossing and selfing ex- 

 periments call for a hypothesis that will account for no 

 stimulation being offered the tubes from the plant 's own 

 pollen, while at the same time great stimulation is given 

 the tubes from the pollen of nearly every other plant. 



This is a straight mathematical problem, and it is 

 hardly necessary to say that it is insoluble by a strict 

 Mendelian notation such as Correns sought to give. This 

 is obvious to any one familiar with the basic mathematics 

 of Mendelism. On the other hand, a near Mendelian in- 

 terpretation satisfies every fact. 



Let us assume that different hereditary complexes stim- 



