212 THE AMERICAN NATURALIST. [Vor. XXXVI. 
rhizome of Dentaria bulbifera, which when taken from the earth 
is as white as ivory, but, if placed in a glass of water and exposed 
to the light of the sun, in a few days turns to a deep violet. 
The cultivated varieties of the genus Mathiola (stock, or gilly- 
flower), from Europe, present a wide range of colors, — as white, 
yellow, red, violet,.and blue. The colors of Iberis (candytutt) 
are white, red, and purple; and the marginal flowers of this 
genus and .of Alyssum and Dentaria have the outer petals 
enlarged, as occurs in the Umbelliferze. -= Hesperis tristis has 
dark-coloréd flowers, visited. at night by insects, which are 
attracted by the scent... | 
The inflorescencé is in umbel-like racemes, which in fruit 
become greatly elongated. The central green buds and the 
surrounding flowers form a nearly flat surface and present a 
variety of color contrasts. Kerner has described several differ- 
ent classes. In Draba verna and. Thlaspi. rotundatum the 
green buds in the center are surrounded by two colored rings, — 
an inner one of small white flowers and an outer one of brown 
or purplish fruits, to which the petals, now enlarged to twice 
their original size, closely adhere. A variegated effect is thus 
produced. In Thlaspi alliaceum and T. arvense the flowers of 
the inner ring are white, but in the outer ring the green sepals 
have changed to yellow, while the fruit remains green. In a 
third group the inner ring is composed of flowers with colored 
petals, which in the outer ring have faded to white. In Draba 
aizoides there is a change from yellow to white, and in Arabis 
cerulea from blue to white. A fourth group is represented by 
A: thionema grandiflorum, in which the upper and under sides of 
the petals are differently colored. The flowers of the inner ring 
present the white upper side of the petals, but in the outer ring 
the petals have changed their position so that the red underside 
is now visible. 
While the flowers of the Cruciferae show that conspicuousness 
is correlated with the visits of insects, they furnish little evidence 
that particular colors have been evolved by the selective tastes 
of special insect groups. Both yellow and white flowers are 
ee xi Syrphidæ, and they are also very frequent visitors to 
the inconspicuous but scented flowers of Lepidium sativum. 
