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THE AMERICAN NATURALIST [Vol. XL VIII 



petals, all other features of the stem, foliage, flowerspikes and 

 fruits being exactly those of the Dutch 0. biennis L. It must 

 certainly be looked upon as a mutation from the 0. biennis L. of 

 our sand dunes. Until now it has only been found a couple of 

 times in single individuals in the midst of the ordinary 0. biennis, 

 the first time in 1900 by Dr. Ernst de Vries in the dunes in the 

 neighborhood of Santpoort, Holland, in one individual — and 

 from this one specimen all the subsequent generations of 0. bien- 

 nis cruciata in the cultures grown by de Vries and by myself have 

 been derived. Besides this, our 0. biennis and 0. bien n 

 are so similar to one another except for floral structure that 

 plants of both types can not be separated before the flowers open. 

 Therefore we have the right to assume that the crossing of these 

 two forms is concerned alone with the floral characters and that 

 with respect to all other characters parents as well as hybrids are 

 mere biennis. Therefore the two variants which arose in my cul- 

 tures from crosses between 0. biennis and 0. biennis cruciata 

 obviously prove the faculty of mutation in 0. biennis. 



In the second place I have found now that it is not necessary 

 to cross O. biennis with 0. bit nnis cruciata in order to obtain the 

 above named mutants, as Davis seems to believe. Already in his 

 new book Professor de Vries figures a dwarf derived from 0. 

 biennis cruciata grown in pure line. Shortly afterwards I myself 

 obtained six mutants from the 0. 'biennis of our sand dunes grown 

 also in pure line. A few details about these cultures of last year 

 may be given here. In all they counted 920 individuals, 430 of 

 which belonged to the third and 490 to the fourth generation of 

 a pure line, the point of departure for which had been one in- 

 dividual brought into the experimental garden in the rosette 

 stage from the dunes near Wyk aan Zee in the beginning of 1905 

 and self-fertilized in the same year. The six mutants which ap- 

 peared in these pure cultures of O. biennis were the following. 

 First a dwarf, then a 'bit nnis sr»>i-<j><fus having 21 chromosomes 

 and finally four individuals of the O. biennis sulfurea, a pale- 

 flowered form of O. biennis, which had been found already several 

 times in our dunes in the midst of the ordinary biennis, but was 

 not with certainty known to be a mutant from the latter form 

 until now. The two first named mutants and one sulfurea ap- 

 peared in the third generation of our pure line, the nanella and 

 the stmi-fjigas coming from the same mother. The three remain- 

 ing 5«7/j/rea-individuals appeared in the fourth generation, all 



