) These latter plants cannot fail to
act on each other like dimorphic or trimorphic species, in which the
union of two individuals is necessary for full and normal fertility.
With ordinary hermaphrodite species, the expansion of only a few flowers
at the same time is one of the simplest means for favouring the
intercrossing of distinct individuals; but this would render the plants
less conspicuous to insects, unless the flowers were of large size, as
in the case of several bulbous plants. Kerner thinks that it is for this
object that the Australian Villarsia parnassifolia produces daily only a
single flower. (10/33. 'Die Schutzmittel' etc page 23.) Mr. Cheeseman
also remarks, that as certain Orchids in New Zealand which require
insect-aid for their fertilisation bear only a single flower, distinct
plants cannot fail to intercross. (10/34. 'Transactions of the New
Zealand Institute' volume 5 1873 page 356.)
Dichogamy, which prevails so extensively throughout the vegetable
kingdom, much increases the chance of distinct individuals
intercrossing. With proterandrous species, which are far more ccommon
than proterogynous, the young flowers are exclusively male in function,
and the older ones exclusively female; and as bees habitually alight low
down on the spikes of flowers in order to crawl upwards, they get dusted
with pollen from the uppermost flowers, which they carry to the stigmas
of the lower and older flowers on the next spike which they visit.
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