Disease In Plants Part 18

_Spiral roots_ occur occasionally in pot plants.

_Flattened roots_ usually result from compression between rocks, the young root having penetrated into a crevice, and been compelled to adapt itself later. The distortions of stems by constricting climbers, wire, etc., have been described, and fruits--_e.g._ Gourds--are easily distorted by means of string tied round them when young.

Distortions of leaves are very common, and are sometimes teratological--_i.e._ due to no known cause--_e.g._ the pitcher-like or hood-like _cucullate_ leaves of the Lime, Cabbage, _Pelargonium_, etc., and of fused pairs in _Cra.s.sula_. Also coherent, bifurcate, crested, displaced and twisted leaves occasionally met with, and in some cases fixed by cultivation, may be placed in this category.

_Puckers_ must be distinguished from pustules, since they consist in local upraisings of the whole tissue, not swellings--_e.g._ the yellowish green pockets on Walnut leaves, due to _Phyllereum_.

Puckered leaves in which the area of mesophyll between the venation is increased by rising up in an arched or dome-like manner are sometimes brought about by excessive moisture in a confined s.p.a.ce.

_Leaf-curl_ is a similar deformation caused by fungi, such as _Exoascus_ on Peaches.

Wrinkling or puckering of leaves is also a common symptom of the work of Aphides--_e.g._ Hops.

Characteristic curling and puckering, with yellow and orange tints, of the terminal leaves of Apples, Pears, etc., are due to insects of the genera _Aphis_, _Psylla_, etc.

Small red and yellow spots with puckerings and curlings of the young leaves of Pears, the spots turning darker later on, are due to _Phytoptus_.

_Leaf-rolling._--The leaves of Beeches, Poplars, Limes, and many other plants, instead of opening out flat, are often rolled in from the margins, or from the apex, by various species of _Phytoptus_, _Cecidomyia_, or other insects, which puncture or irritate the epidermis in the young stages and so arrest its expansion in proportion to the other tissues. According as the lower or upper surface is attacked the rolling is from the morphologically upper surface downwards, or _vice versa_. Very often the mesophyll is somewhat thickened where rolled and _Erineum_-like hairs may be developed--_e.g._ Lime. Many caterpillars also roll leaves, drawing the margins inward to form shelters--_e.g._ _Tortrix viridana_, the Oak leaf-roller. Certain beetles--_Rhynchitis_--also roll up several leaves to form a shelter in which the eggs are laid.

Webs are formed among the mutilated leaves of Apples by the caterpillars of _Hyponomeuta_.

It must be borne in mind that instances can be found of teratological change of every organ in the plant--_e.g._ stamens transformed into carpels or into petals; anthers partly polliniferous and partly ovuliferous; ovules producing pollen in their interior, and so on, being simply a few startling examples of what may happen. Such abnormalities are frequently regarded as evidence of internal causes of disease, and this may be true in given cases; in a number of cases investigated, however, it has been shown that external agents of very definite nature bring about just such deformations as those sometimes cited as examples of teratology due to internal causes, and the question is at least an open one whether many other cases will not also fall into this category.

The study of galls has shown that insects can induce the formation of not only very extraordinary outgrowths of tissues and organs already in existence, but even of new formations and of tissue elements not found elsewhere in the plant or even in its allies; and Solms' investigations on _Ustilago Treubii_ show that fungi can do the same, and even compel new tissues, which the stimulating effects of the hyphae have driven the plant to develop, to take part in raising and distributing the spores of the fungus--_i.e._ to a.s.sume functions for the benefit of the parasite.

Molliard has given instances of mites whose irritating presence in flowers causes them to undergo teratological deformations, and Peyritsch has shown that the presence of mites in flowers induces transformations of petals into sepals, stamens into petals. Similarly De Bary, Molliard, Magnus, Mangin, and Giard have given numerous cases of the transformation of floral organs one into another under the irritating action of fungi, of which the transformation of normally unis.e.xual (female) flowers into hermaphrodite ones, by the production of stamens not otherwise found there, are among the most remarkable.

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 p.r.i.c.king the centre of the torus, and Molliard got double flowers by mechanical irritation.

NOTES TO CHAPTER XXVII.

For the details and cla.s.sification of the mult.i.tude 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, "Cecidies 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--Advent.i.tious 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.

_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 acc.u.mulated during the period of active a.s.similation. In such cases the axillary buds, which would normally have pa.s.sed 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 _prevent.i.tious 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 a.s.similatory 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 subst.i.tution branches are obtained when a leader is broken or cut away.

_Advent.i.tious 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 advent.i.tious buds on roots.

Advent.i.tious buds are also common on leaves (_e.g._ _Bryophyllum_, Ferns, etc.), and are frequently induced on them by wounds--_e.g._ _Gesneria_, _Gloxinia_, etc. Even cut cotyledons may develop them, and pieces of leafless inflorescence (Hyacinth), hypocotyl (_Anagallis_), and in fact practically any wounded tissue with a store of reserve materials may be made to develop them: thus they have been found arising from the pith of Sea-kale, and are commonly developed from the cut bulb scales of Hyacinths.

_Apospory_ and _Apogamy_ are particular cases of the production of vegetative buds on the leaves in place of sporangia in Ferns (Apospory), and on prothallia in place of Archegonia (Apogamy), in the latter case induced by dry conditions and strong illumination.

NOTES TO CHAPTER XXVIII.

In addition to the literature quoted in the notes to Chapter XXVII., the student should consult the works on Forest Botany for the scattered information regarding advent.i.tious 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.

CHAPTER XXIX.

GRAFTS.

_Grafting--Comparison with cuttings--Effects of environment-- Relations between scion and stock--Variation in grafts-- Grafting and parasitism--Infection--Pollination--Grafts-hybrids --Predisposition of Natural grafts--Root-fusions._

Grafting is a process which consists in bringing the cambium of a shoot of one plant into direct union with that of another, and is practised in various ways, the commonest of which is as follows:

One plant--the _stock_--rooted in the ground, is cut off a short distance above the surface of the soil, and a shoot from the second plant--the _scion_--cut off obliquely with a sharp knife, is inserted into a cleft in the stock, so that the two cambiums (and sometimes the cortex and pith of each as well) are in close contact: the scion is then tied in position, the wounds covered with grafting wax, and the whole left until union of the tissues is completed. This union depends on the formation of _callus_ at the cut surfaces, and the intimate union of the ingrowing cells from each callus.

The development of the callus follows the course described for wounds, cuttings, etc., and the union is exactly comparable to the union of the two lips of a healing callus over a wound (see p. 197).

Grafting was known and practised far back in the ages. Virgil was well acquainted with the process, and Theophrastus compared it with propagation by cuttings.