2 SILVA OF NORTH AMERICA. LILIACES. 
erect or rarely pendulous panicles, or occasionally in simple racemes or spikes ; panicles nearly sessile or 
raised on short or elongated peduncles furnished with leaf-hke bracts ; bracts of the inflorescence ovate, 
acute or acuminate, concave, thick and fleshy, white, often more or less tinged with purple, decreasing 
in size from below upward, those subtending the pedicels thin and scarious; pedicels in two or three- 
flowered clusters, or single at the base of the panicle, simple or rarely forked near the middle, shorter 
than the flowers, curved, slightly spreading, pendulous, ebracteolate. Perigone cup-shaped, composed 
of six segments in two series, more or less united at the base into a short tube, expanding in the 
evening for a single night, marcescent ;* segments thick, ovate-lanceolate, creamy white or white tinged 
with green, and often flushed with purple on the back, usually furnished at the apex with small tufts 
of white hairs, those of the outer rank narrower, shorter, and more colored than the more delicate and 
petaloid segments of the inner rank. Stamens six, in two series, hypogynous, free or adnate to the 
base of the segments of the perigone, usually shorter than the ovary, white; filaments clavate, fleshy, 
obtuse and slightly three-lobed at the apex, covered, especially above the middle, with one-celled 
transparent hairs, or acute at the apex and glabrous (Hesperoyucca), erect before anthesis, becoming 
recurved after the maturity of the anthers; anthers sagittate or cordate at the base, rounded, entire or 
emarginate at the apex, glabrous, or furnished with tufts of apical hairs (Hesperoyucca), attached on 
the back, introrse, two-celled, the cells opening longitudinally, curling backward, and expelling the 
large globose powdery or agglutinated pollen-grains. 
peculiar moths which are necessary to insure the fertilization of 
the flowers of these plants. This is Yucca aloifolia of the south- 
eastern United States, whose flowers are better adapted than those 
of the other species for self-fertilization. The stigmatic lobes are 
sessile, and the stigmatic liquid is abundant, often overflowing 
from the stigmatic tube, so that there is a chance that the pollen- 
grains may be earried accidentally from the comparatively longer 
stamens to the stigmatic secretions, or may fall on the papillose 
apex of the stigmatic lobes ; and this species often fructifies abun- 
dantly in cultivation in regions where the Yucca Moth does not 
exist, although in a case where the flowers were protected from the 
visits of all insects they were not fertilized. (See Riley, Rep. Mis- 
sourt Bot. Gard. iii. 118.) 
So far as is now known, the flowers of all the species of the 
region east of the Rocky Mountains are fertilized by females of a 
nocturnal moth, Pronuba yuccasella, Riley (Trans. St. Louis Acad. 
iii. 56, f. 2) ; in California the flowers of Yucca arborescens are fer- 
tilized by Pronuba synthetica, Riley (i. c. 141, t. 41, f. 1, 2, t. 43, £. 
1), and those of Yucca Whipplei by Pronuba maculata, Riley (Proc. 
Am. Ass. Adv. Sct. xxix. 633 [1881] ; Rep. Missouri Bot. Gard. iii. 
139, t. 42,.f. 2). When the perigone opens in the evening, the 
anthers split and discharge the pollen-grains which adhere in the 
anther slits. The female moth now enters the flower and begins to 
gather the pollen with her peculiar maxillary prehensile spinose 
tentacles, visiting the stamens in turn, and forming a ball of pollen 
often three times as large as her head. When this is completed she 
visits a flower of another plant, selecting one that has just opened or 
which had opened during the previous night, and bearing her load 
of pollen held by the rolled-up palpi below and close to the head. 
In entering the flower she brings her abdomen against the pistil, 
with the body between two of the stamens, which she straddles 
with her legs, the head being usually turned toward the stigma, 
and in this position she pierces the ovary obliquely just below the 
middle, and deposits an egg with her ovipositor in the ovarian cell 
next the placenta. When the egg is deposited she slowly with- 
draws her ovipositor, and then runs to the tip of the pistil and 
pushes the ball of pollen collected in another flower down into the 
stigmatic tube. This operation she usually performs after deposit- 
Ovary superior, sessile or rarely stipitate, 
ing each egg, although two or three eggs are occasionally laid 
The larva 
hatches at the end of a week, and soon enters one of the developing 
ovules by its funicular base ; it matures with the ripening of the 
seeds, of which it has destroyed a dozen or less during its growth, 
and just before the fruit ripens it bores its way out, and reaches the 
ground, probably by the aid of a silken thread. Penetrating the 
soil to the depth of several inches, the larva then spins a tough 
silky cocoon, in which it remains until a few days before the 
before the pollen is transferred to the stigmatic tube. 
Yuccas bloom in the following year, when it is transformed into a 
chrysalis armed on the head with an acute spine, and on the back 
with spatulate spines, by means of which it works its way to the 
surface of the ground, and the moth emerges. This wonderful cor- 
relation between the insect and the flower is all the more remark- 
able because the insect does not derive any direct benefit from it. 
She does not visit the flower in search of food and incidentally 
transfer the pollen from the stamens to the stigma. The flowers 
of Yucca either produce no nectar or produce it in the smallest 
quantity, and the Pronuba does not feed upon the pollen. Her 
labors appear to be purely maternal, and her only object in gather- 
ing the pollen of one flower and thrusting it down the stigma of 
another seems to be the development of the seeds which are to sup- 
ply her offspring with food. The action of this insect in fertilizing 
the flowers of Yucca, first noticed by Dr. George Engelmann, has 
been carefully studied by Professor C. V. Riley, and by Professor 
William Trelease, who has visited many of the species of Yucca in 
their native homes for the purpose of watching their pollination. 
(See Engelmann, Bull. Torrey Bot. Club, iii. 33; Trans. St. Louis 
Acad. iii, 28. — Riley, Trans. St. Louis Acad. iii. 55 ; Rep. Missouri 
Bot. Gard. iu. 99.— Kerner von Marilaun, Pflanzenleben, ii. 155, 
f. 1-5. — Trelease, Rep. Missouri Bot. Gard. iv. 181.) 
1 The flower of Yucca expands during the evening into a more 
or less widely opened bell, and soon after sunrise the next morning 
begins to close by the gradual bending in of the points of the 
segments, which, during the next two or three days, form a bladder- 
like perianth with six broader or narrower openings between the 
segments, and then withers and dries up. 
