(10/29. 'Die Befruchtung' etc. page 311.) In
order that distinct plants should be intercrossed, it is of course
indispensable that two or more individuals should grow near one another;
and this is generally the case. Thus A. de Candolle remarks that in
ascending a mountain the individuals of the same species do not commonly
disappear near its upper limit quite gradually, but rather abruptly.
This fact can hardly be explained by the nature of the conditions, as
these graduate away in an insensible manner, and it probably depends in
large part on vigorous seedlings being produced only as high up the
mountain as many individuals can subsist together.
With respect to dioecious plants, distinct individuals must always
fertilise each other. With monoecious plants, as pollen has to be
carried from flower to flower, there will always be a good chance of its
being carried from plant to plant. Delpino has also observed the curious
fact that certain individuals of the monoecious walnut (Juglans regia)
are proterandrous, and others proterogynous, and these will reciprocally
fertilise each other. (10/30. 'Ult. Osservazioni' etc. part 2 fasc 2
page 337.) So it is with the common nut (Corylus avellana) (10/31.
'Nature' 1875 page 26.), and, what is more surprising, with some few
hermaphrodite plants, as observed by Hermann Muller. (10/32. 'Die
Befruchtung' etc. pages 285, 339.
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