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.) 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.  The degree to which distinct plants will thus be intercrossed depends on the number of spikes in full flower at the same time on the same plant.  With proterogynous flowers and with depending racemes, the manner in which insects visit the flowers ought to be reversed in order that distinct plants should be intercrossed.  But this whole subject requires further investigation, as the great importance of crosses between distinct individuals, instead of merely between distinct flowers, has hitherto been hardly recognised.