Proliferation consists in the unexpected and abnormal on-growing or budding out of parts- stems, tubers, flowers, fruits, etc.-which in the ordinary course of events would have ceased to grow further or to bear buds or leaf-tufts directly. Thus we do not expect a Strawberry - the swollen floral axis - to bear a tuft of leaves terminally above the achenes, but it occasionally does so, and similarly Pears may be found with a terminal tuft of leaves, Roses with the centre growing out as a shoot, Plantains (Plantago) with panicles in place of simple spikes, and so on.
We regard such cases as teratological, because they are exceptional for the particular species, and as pathological because they appear to be connected with over-feeding in soils with excessive supplies of available food-materials; but it should be noted that conditions quite comparable to proliferation are normal in the inflorescences of Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of proliferations come under the head of injurious actions of fungi, insects, and other agents.
Proliferation of tubers is sometimes seen in Potatoes still attached to the parent plant in wet weather following a drought. The eyes grow out into thin stolons, or forthwith into new tubers sessile on the old tuber. Similarly in store we sometimes find the eyes transformed directly into new tubers, and cases occur where the growth of the eye is directed backwards into the softening tuber, and a small potato is formed inside the parent one.
Threading is also occasionally met with in the "sets" when ripened too rapidly in hot dry soils.
Vivipary is a particular case of proliferation, in a certain sense, where the seeds appear to germinate in situ, and we have small plants springing from the flowers, reminding us of wheat which has sprouted in the shocks in damp weather. In reality, however, the grains are here replaced by bulbils which sprout before they separate from the inflorescence. In varieties of Poa, Polygonum, Allium, Gagea, etc., this phenomenon is constant in plants growing in damp situations.
In addition to the literature quoted in the notes to Chapter 27., the student should consult the works on Forest Botany for the scattered information regarding adventitious buds. A good account may be found in Busgen, Bau und Leben unserer Waldbaume, Jena, 1897.
For Apospory and Apogamy, see Lang "On Apogamy and the Development of Sporangia upon Fern Prothalli," Phil. Trans., vol. 190, 1898, p. 187, where the literature is collected.