Disease in plants

by H. Marshall Ward

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.

Prolepsis.—It frequently happens that branches or whole plants are suddenly defoliated in summer,—e.g. by caterpillars or other insects—at a time when considerable stores of reserves had already been accumulated during the period of active assimilation. In such cases the axillary buds, which would normally have passed into a dormant condition over the winter had the leaves lived till the autumn-fall, suddenly shoot out into proleptic shoots (also termed Lammas shoots), and reclothe the tree with foliage. The wood of the year in which this occurs may exhibit a double annual ring, and the vigour of the tree is likely to suffer in the following season and no fruit be matured.

Proleptic branches may also be due to the shooting out of accessory buds—i.e. extra buds found in or near the leaf-axils of many plants, such as Willow, Maples, Cercis, Robinia, Syringa, Aristolochia, etc.—which do not normally come to anything, or do so only if a surplus of food materials is provided.

Dormant buds, or preventitious buds, are such as receive no sufficient supply of water and food materials to enable them to open with the other buds in ordinary years, for in most trees only the upper buds on the branches develop into new shoots. The lower buds do not die, however, but merely keep pace with the growth in thickness of the parent branch, and may be elongated sufficiently each year to raise the minute tips level with the bark, their proper cambium only remaining alive but not thickening the bud.

When, by the breaking of the branch above the insertion of the dormant bud—or by pruning, defoliation by insects, etc.—the transpiration current and supplies of food materials are in any way deflected to the minute cambium and growing points of the dormant buds, they are stimulated to normal growth, and may grow out as epicormic shoots or "shoots from the old wood." In many cases such epicormic shoots are stimulated to grow out by suddenly exposing an old tree to more favourable conditions of root-action and assimilatory activity, owing to the felling of competing trees which previously hemmed it in from light and air, and restricted the spread and action of its roots in the soil. This is often seen in old Elms, Limes, etc.

It is by such means as the above that substitution branches are obtained when a leader is broken or cut away.

Adventitious buds are such as are newly formed from callus or other tissues in places not normally provided with buds, as is often seen on occluding wounds—e.g. stool shoots. They may also be developed on roots, a fact utilised in propagating Bouvardias, Horse-radish, etc., by means of root-cuttings, and the suckers of Plums and other fruit trees are shoots springing from adventitious buds on roots.