The fertilisation of one of the higher plants depends, in the first
place, on the mutual action of the pollen-grains and the stigmatic
secretion or tissues, and afterwards on the mutual action of the
contents of the pollen-grains and ovules. Both actions, judging from the
increased fertility of the parent-plants and from the increased powers
of growth in the offspring, are favoured by some degree of
differentiation in the elements which interact and unite so as to form a
new being. Here we have some analogy with chemical affinity or
attraction, which comes into play only between atoms or molecules of a
different nature. As Professor Miller remarks: "Generally speaking, the
greater the difference in the properties of two bodies, the more intense
is their tendency to mutual chemical action...But between bodies of a
similar character the tendency to unite is feeble." (12/15. 'Elements of
Chemistry' 4th edition 1867 part 1 page 11. Dr. Frankland informs me
that similar views with respect to chemical affinity are generally
accepted by chemists.) This latter proposition accords well with the
feeble effects of a plant's own pollen on the fertility of the
mother-plant and on the growth of the offspring; and the former
proposition accords well with the powerful influence in both ways of
pollen from an individual which has been differentiated by exposure to
changed conditions, or by so-called spontaneous variation.
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