1910] ANDREWS—TWIN HYBRIDS 195 
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erally more or less united at the top, and somewhat pubescent. 
The calyx tube is a little longer than the sepals. The cell walls of 
the epidermis of the sepals of O. Lamarckiana, especially the inner 
| epidermis, are very irregular and zigzag in form. At the angles of 
these zigzag places the walls of the inner epidermis are marked by 
conspicuous thickenings. In the case of the outer epidermis the 
thickenings at these places extend away from the main wall as 
. T-shaped processes. The same is true, but to a slightly less degree, 
for the inner and outer epidermis of the petals. The four very large, 
| broadly obovate, entire petals are very conspicuous, and almost or 
quite equal the length of the calyx tube. One of the distinguishing 
features of O. Lamarckiana is the tall stigma, which far exceeds 
in height the stamens, and on this account self-pollination can- 
not occur. The anthers of O. Lamarckiana open in damp or 
rainy weather as well as on clear days, but this, as we shall see 
later, is not true of some forms, as for example the twin hybrids. 
: This can b2 accomplished in the parent species, O. Lamarckiana, 
because at maturity the tissues of the cell walls of the anther are 
completely differentiated and become free, so that the outer wall may 
open out in the usual way. 
The capsules are four-celled and open by as many valves at 
maturity. The pubescence on the capsules is not so dense as on the 
ovary. As the old flowers disappear and new ones appear, long 
rows of capsules are left on the older parts of the spike. 
Oenothera biennis is not as stout as O. Lamarckiana. Its stem is 
erect, cylindrical, branching, and sparsely covered with coarse but 
non-glandular hairs. It is branched, is about 1 cm. in diameter, and 
I~I.5 meters in he’ght. 
The leaves are dark green, broad (somewhat more so than in 
O. Lamarckiana), about three times as long as wide, with margins 
slightly wavy, and all of the stem leaves are petioled. In the last 
two respects the Holland species is somewhat different from the more 
harrow-leaved American species of O. biennis, which has only the 
“lowest petioled’’s and the margins “repand-denticulate.”’ ‘These 
are among the differences that show that the form O. biennis, which 
_ 'S very common in the United States, is not the same as the O. biennis 
‘ BRITTON AND Brown, Flora of the northern U.S, and Canada 2:486. 
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