408 Origin of Each Species Considered Separately. 



but above them they were variegated. The gall stimulus, 

 therefore, does not exert its influence only on those qual- 

 ities which are necessary for the formation of the galls, 

 but on others as well. 



The effect of a single mutation on the most diverse 

 and important, as well as on subsidiary, qualities may 

 be of a similar nature as that of a gall stimulus. But if 

 it is difficult to discover the chemico-physiological nature 

 of the gall stimulus ; it is wellnigh impossible to penetrate 

 into the mystery of the chemical nature of a primary 

 mutation. 



Let us now make a more detailed study of the char- 

 acters of our Oenothera lata and let us begin with the 

 stamens. The anatomical structure of these has been in- 

 vestigated by Prof. J. PoHL^ (Fig. 90), partly on the 

 plants of my first /a /a- family (p. 285) in 1894, partly on 

 a larger culture which I raised in that year from the 

 seeds of the second /a/a-family (seeds of 1889 and of 

 1890, see p. 288) and partly on isolated new mutations. 

 The structure of the stamens was the same in all cases; 

 and was, therefore, independent of the ancestry of the 

 plant. 



Pollen formation takes place in Oenothera Lam arc- 

 kiana and O. lata in the ordinary way ; the mother cells 

 enclosed in the tapetum each divide into two daughter 

 cells and each of these into two granddaughter cells. The 

 loculus increases in capacity by the dissolution of the 

 tapetum ; and the further development of the pollen- 

 grains takes place in the fluid which now surrounds them. 

 The ripe pollen of 0. Laniarckiana consists of two forms 



^Julius Pohl, Ueher Variationsweite von Oenothera Lamarc- 

 kiuna; Oesterr. Botan. Zeitschr., 1895, Nos. 5 and 6, and Plate X. — 

 See also R. R. Gates^ Pollen Development in Hybrids of Oenothera 

 lata; in Botanical Gazette, T. 43, p. 81. (Note of 1908.) 



