These and similar examples suffice to awaken doubts as to whether any teratological change really arises "spontaneously," especially when we learn how slight a mechanical irritation of the growing point may induce changes in the flower; _e.g._ Sachs showed that a sunflower head is profoundly altered by pricking the centre of the torus, and Molliard got double flowers by mechanical irritation. NOTES TO CHAPTER XXVII. For the details and classification of the multitude of facts, the student is referred to Masters' _Vegetable Teratology_, Ray Society, 1869, and the pages of the _Gardeners' Chronicle_ since that date. Concerning torsions, etc., the student should read De Vries, "On Biastrepsis in its Relation to Cultivation," _Ann. of Bot._, Vol. XIII., 1899, p. 395, and "Hybridising of Monstrosities," _Hybrid Conference Report_, _Roy. Hort. Soc._, 1900, Vol. XXIV., p. 69. The reader will find an excellent account of the abnormalities in flowers due to the action of parasitic insects and fungi in Molliard, "Cécidies Florales," _Ann. des Sc. Nat._, Ser. VIII., Bot., T. 1, 1895, p. 67. CHAPTER XXVIII. PROLIFERATIONS. _Proliferations--Vivipary--Prolepsis--Lammas shoots--Dormant buds--Epicormic shoots--Adventitious buds--Apospory and apogamy._ _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.