No. 423.] WORTHERN POLYPETALQUS FLOWERS. 231 
have rarely observed visitors to the white and blue varieties, but 
on the yellow V. rotundifolia, which blooms early in May, I 
have seen many bees belonging to the genera Bombus, Nomada, 
and Andrena. Besides the conspicuous flowers, which are often 
unfruitful, many species produce cleistogamic flowers, in which 
the petals are reduced to. mere scales, but the green sepals 
remain essentially unchanged. Occasionally the violets bloom 
a second time in late fall. "n 
The failure of the yellow plastids to develop and the predomi- 
nance of the colored cell sap produce blue flowers, and the non- 
development of both color elements results in white blossoms. 
In these color changes other ecological factors are more impor- 
tant than insects. For instance, of the yellow-flowered species 
one is visited by flies, another by bees, and the either yellow or 
blue V. calcarata is adapted to Lepidoptera. 
The passion flowers are mostly natives of South America and 
are fertilized by humming birds and bumblebees. The calyx 
of the common Passiflora caerulea remains green until it has 
attained nearly its full size, when it changes to blue and white; 
the petals are white; the outer corona consists of several rows 
of blue filaments banded with white, but the inner corona is 
smaller and unmarked. Fritz Müller considered the corona to 
be of service in detaining small insects and keeping them caged 
for humming birds. 
The Cacti, of which over a thousand species are natives of 
America, are especially abundant on the sandy plains of Mexico. 
The nearest living representative of the ancestral stock of this 
family is the genus Pereskia, which still possesses leaves of the 
usual form. “The earliest derived line was Opuntia. From 
the primitive Opuntia forms the columnar Cereus line was 
derived, with its numerous generic branches and diverse habits. 
Low down upon the columnar Cereus line the Echinocactus line 
branched out, which gave rise later to Mamillaria and still later 
to Anhalonia” (Botanical Gazette, Vol. XXVII, No. 3, p. 228). 
The flowers of the Cacti are solitary and sessile, with the sepals 
and petals and stamens numerous and spirally arranged. In size 
they are usually large and showy, as in Cereus grandiflora, queen 
of the night, where they are 20 cm. in diameter. In Pereskia 
