84 MISSOURI BOTANICAL GARDEN BULLETIN 
shaped and nearly white—having a tint of green and some- 
times of purple on the outside. A great number of these 
flowers, throughout the panicle, are open at the same time. 
As old ones wither, new ones open to take their places, thus 
presenting a full and beautiful display throughout the 
flowering period. During the day the new flowers are only 
very slightly opened; the floral parts being incurved into 
a more or less spherical body about one and one-half inches 
in diameter. However, toward evening, they expand into 
broad bell-form, exposing the white inner surface as well as 
the stamens and pistil. So it is at twilight that the plant 
appears in its greatest glory, and it is at this time, too, that 
the flowers give off their sweet, delicate perfume. The 
flowers remain open all night, but close up by sunlight, 
never to open again. 
There are a considerable number of species of Yucca in 
the Garden collection; a few of which are from the western 
plains, being hardy and blossoming early; the majority 
are natives of warmer regions, however, and must neces- 
sarily be housed during our cold season. These are later in 
their various periods of flowering. Of the hardy western 
species there are several plants in different parts of the 
Garden. The largest and oldest of these is a splendid cluster 
near the center of the main garden. This was planted by 
Mr. Shaw, in 1860, in memory of his very near friend and 
medical advisor, Dr. Charles A. Pope, formerly one of the 
leading physicians of St. Louis. This plant, Yucca angusti- 
folia, is reported to be the first one of the species introduced 
into cultivation. 
It is not intended to imply that Yuccas are strong com- 
petitors for honor in a floral display, but the aim is rather 
to direct the attention of visitors to interesting phenomena 
in connection with the life history of these plants, namely 
a study of the interrelations between the Yuccas and cer- 
tain insects—how each is entirely dependent on the other 
for its existence; how if the plants should all perish the 
insects likewise would soon die, and if the insects should by 
some means become extinct, it must inevitably follow that 
the plants, also, would sooner or later become exterminated. 
Let us consider a few of the problems that confronted the 
early investigators of this subject and then follow briefly 
the lines of reasoning that lead to the discovery of some of — 
these most remarkable facts. One of the first students of 
