No. 423.] NORTHERN POLYPETALOUS FLOWERS. 21I 
and in the situation of the anthers in relation to these and to 
the stigma. Many genera show evidences of retrogression in 
the small white flowers and in the regularity with which self- 
fertilization occurs. Pringlea antiscorbutica, which grows on 
the stormy shores of Kerguelen Land, where no winged insects 
can exist, since they would be swept into the sea, has reverted 
to wind-fertilization. In sheltered places the. petals, which 
usually are wanting, are frequently present, and “on the same 
raceme some flowers may possess only a single petal, others two, 
three, or four ; and the petals, though usually of a pale greenish 
color, are occasionally adorned with purple." This family is 
much less attractive to insects than the Umbelliferz, and is 
visited chiefly by flies and the less specialized bees, such as 
Andrena and Halictus, and by a few beetles and Lepidoptera. 
Some flowers have no recorded visitors. 
Of the 113 flowers, 2 are green, 54 white, 46 yellow, 1 red, 
and ro purple; 20 of the 37 genera contain white and 16 yellow 
flowers, but only 4 genera contain both yellow and white. 
The white flowers are usually small, or even minute, and self- 
fertilized as in Subularia (awlwort), Lepidium (cress), and Bursa 
(shepherd's purse). The yellow flowers are larger, as in Sinapis 
and Brassica, and frequently show a tendency to change to 
white. In Rhaphanus (radish) the wild species, A. raphanis- 
trum, is yellow, changing to white; while the garden radish, 
R. sativus, is white or pink. In Roripa (nasturtium) the two 
aquatic species are white, as well as the horse-radish, which 
grows wild along streams; the other species are yellow. In 
Draba four of the species are yellow and seven white, and 
D. nemorosa is yellow fading to whitish. In other species the 
calyx changes from green to yellow. It would seem probable 
that in these genera the white-flowered species have been derived 
from yellow progenitors ; yellow appears to have been very early 
developed, . and was not improbably the original color of this 
family. 
The white flowers very frequently show a tendency to turn 
pink or purplish, and three genera contain both purple and white 
flowers. Malcolmia maritima:has pink-red flowers, changing to 
violet purple. -This transition is beautifully illustrated by the 

