In the several species of Viola the various parts of the flowers
have also been modified in very different degrees. Those plants which in their
own country produce flowers of full or nearly full size, but never expand (as
with Thelymitra), and yet set fruit, might easily be rendered cleistogamic.
Lathyrus nissolia seems to be in an incipient transitional state, as does
Drosera Anglica, the flowers of which are not perfectly closed. There is good
evidence that flowers sometimes fail to expand and are somewhat reduced in size,
owing to exposure to unfavourable conditions, but still retain their fertility
unimpaired. Linnaeus observed in 1753 that the flowers on several plants brought
from Spain and grown at Upsala did not show any corolla and yet produced seeds.
Asa Gray has seen flowers on exotic plants in the Northern United States which
never expanded and yet fruited. With certain English plants, which bear flowers
during nearly the whole year, Mr. Bennett found that those produced during the
winter season were fertilised in the bud; whilst with other species having fixed
times for flowering, but "which had been tempted by a mild January to put forth
a few wretched flowers," no pollen was discharged from the anthers, and no seed
was formed. The flowers of Lysimachia vulgaris if fully exposed to the sun
expand properly, while those growing in shady ditches have smaller corollas
which open only slightly; and these two forms graduate into one another in
intermediate stations.
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