Allowing for all the circumstances which are
known to prevent the production of seed, such as too great luxuriance of
foliage, too little or too much heat, or the absence of insects to
cross-fertilise the flowers, Mr. Darwin shows that many species which
grow and flower with us, apparently in perfect health, yet never produce
seed. Other plants are affected by very slight changes of conditions,
producing seed freely in one soil and not in another, though apparently
growing equally well in both; while, in some cases, a difference of
position even in the same garden produces a similar result.[51]
_Reciprocal Crosses._
Another indication of the extreme delicacy of the adjustment between the
sexes, which is necessary to produce fertility, is afforded by the
behaviour of many species and varieties when reciprocally crossed. This
will be best illustrated by a few of the examples furnished us by Mr.
Darwin. The two distinct species of plants, Mirabilis jalapa and M.
longiflora, can be easily crossed, and will produce healthy and fertile
hybrids when the pollen of the latter is applied to the stigma of the
former plant.
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