295 ^' Ruggles Gates. 



ploidy in O, gigas originates from the chance union of two unreduced 

 or diploid germ cells. In support of this view these investigators 

 announced independently the discovery of tripioid or 3x mutants 

 having twenty-one chromosomes. Stomps found one such mutant 

 from Liunarckiana, which he calls semigigas ; and eleven Hero or Sat 

 mutants from Laiiiarckiana and its mutants rubrinervis and lata 

 pollinated hy cruciata, muricata, biennis or Millersi. The frequency 

 of tripioid forms in these crosses (they were easily recognizable by 

 their larger size and deep green colour) was about three per 

 thousand. But it seems probable that these tripioid mutants have all 

 originated from the fertilization of a 2x (diploid) egg by a normal 

 or haploid pollen-grain, especially as in the reciprocal crosses 

 tripioid forms do not appear to have been found. These crosses do 

 not therefore, as Stomps has supposed, furnish evidence of the 

 occurrence of diploid pollen-grains. The same is true of the eight 

 tripioid mutants obtained by Miss Lutz. They may all have come 

 from 2x eggs fertilized by pollen-grains having x chromosomes. 



There is, furthermore, no observational evidence of the existence 

 of diploid pollen-grains, though the megaspores which have been 

 much less studied have furnished a case (Geerts) of a megaspore 

 mother-cell of Laiiiarckiana having twenty-eight chromosomes. I 

 pointed out (16) the probability that such a cell would develop an 

 embryo after omitting both reduction and fertilization, and that 

 gigas mutants may therefore originate in this manner. It has also 

 been pointed out (20) that the occasional rare pollen-grains of 

 Lamarckiana which, like gigas, have four lobes instead of three, may 

 be diploid in chromosome-content. But there is at present no 

 evidence that such pollen-grains are functional. The exact manner 

 of origin of gigas hence remains uncertain, though it is possible that 

 both methods of origin may occur. The discovery of tripioid 

 mutants, however, indicates the sporadic occurrence of diploid eggs 

 in Laiiiarckiana and its derivatives, though there are of course other 

 conceivable ways in which triploidy might have originated, such as 

 the formation of the embryo from a triple fusion endosperm nucleus. 

 This method is improbable, however, for the (Enothera embryo-sac 

 only contains four nuclei (two synergids, the egg and a polar 

 nucleus) and there is very little endosperm-formation. 



It is highly probable that the exceptional degree of variation in 

 gigas is concerned, at least in part, with changes in the chromosome- 

 number of different individuals. A number of these types have 

 been figured (21, 24). 



Series of mutations which are parallel to those oi Lamarckiana 

 have been obtained by Stomps (29) and by Gates (18) in different 

 races of O. biennis. Stomps obtained two mutants from the Fj of 

 biennis x biennis cruciata. The cruciate variety differs from the 

 normal (from which it has probably originated by a mutation) 

 only in the cruciate character of the flowers. This character 

 behaves as a Mendelian recessive, splitting out in F^. In the F^ 

 appeared one O. mut, biennis nanella (dwarf), and one 0. biennis 

 semigigas which was larger and possessed twenty-one chromosomes, 

 having also long styles unlike biennis races. 



In a race of 0. biennis from the Madrid Botanical Garden, 

 which had evidently undergone crossing (18), many of the plants 

 belonged to types corresponding to Lamarckiana, rubrinervis and 



