Part 17 (2/2)

Such _teratological_ conditions are however by no means always _pathological_: that is to say, they may be variations which do not threaten the existence of the plant. In some cases they are clearly due to exuberant nutrition, and although they may occasionally predispose to disease, in others they show no evidence of doing so. The whole practice of horticulture and agriculture abounds in examples of teratological sports or varieties which are transmissible by seeds, budding and grafting, and other means--_e.g._ double flowers, hypertrophied floral organs (cauliflowers), seedless grapes and oranges, crested ferns, etc.; and even when such varieties could not live as such in a state of nature, there is evidence to show that many of them readily revert to the original seed-bearing or single condition, and adapt themselves to the altered environment.

Every part of the plant may exhibit teratological changes, and I shall for the most part select cases in ill.u.s.tration which indicate approach to pathological states, and group with them cases known to be pathological in origin.

_Atrophy_ is a common phenomenon denoting dwindling or reductions in size of organs due to insufficient nutrition, or arrest of growth from various causes.

Atrophy of leaves is a common result of the attacks of parasitic fungi, even when the latter induce local hypertrophy--_i.e._ excessive growth of particular parts, _e.g._ _Synchytrium_ on Dandelions and Anemones.

_Puccinia suaveolens_ causes partial atrophy of the leaves of Thistles, _Aecidium Euphorbiae_ of those of _Euphorbia_.

The carpels of Anemone are atrophied in plants attacked by _Aecidium_, and the whole flower is suppressed in Cherries infested with _Exoascus Cerasi_, while other fungi--_e.g._ _Cystopus_, _Exoasci_, etc.--cause atrophy of the seeds, and numerous instances of atrophied grain occur in plants infested with Ustilagineae.

Atrophy of the grains of cereals is sometimes due to the direct attack of animals, _e.g._ eel-worms (_Tylenchus_) eat out the grains of Corn; weevils and other beetles (_Curculio_, _Bruchus_, etc.) similarly devour the contents of grain and nuts, the flowers of Peas and Apples, and so forth, inducing atrophy of the parts left. Still more striking cases are afforded by small insects which bore into the halms of cereals, and cause atrophy of the whole ear--_e.g._ _Cephus_ in Wheat and Rye. Barley occasionally withers after flowering, the grain atrophying from no known cause, terms like _consumption_ given to the disease conveying no information.

Atrophy of young fruits is commonly due to the flowers not setting--_i.e._ some agent has interfered with the normal transference of the pollen to the stigma. This may be due to excessive rain was.h.i.+ng out the pollen (_e.g._ Vine), to a lack of the necessary insects which effect pollination, often seen in greenhouse plants; to the stamens being barren--_e.g._ certain varieties of Vine--or to the premature destruction of the stigmas by frost, as in Cherries, Pears, etc., or by insects, as in Apples, or fungi, _e.g._ the infection of bilberries with _Sclerotinia_; or even by poisonous gases, as is sometimes seen in Wheat, etc., growing near alkali works. Drought is also a common cause of atrophy of young Plums.

_Shanking of Grapes_ is a particular case of atrophy and drooping of the immature fruits, due to the supplies being cut off by some agency. It may arise from very various causes which bring about disease in the leaves or roots, and should always be looked upon as a sign of weakness in the Vine, the structure of which is affected, _e.g._ poor wood--or the functions interfered with, _e.g._ water supplies deficient owing to paucity of roots.

Barren Apple, Pear, Plum, and other flowers are often found to have been bored through the petals while in bud, and the whole ”heart” of the flower eaten out by the grubs of _Anthonomus_, leaving the unopened buds brown and dead, as if killed by frost or drought, and often erroneously supposed to be so.

The wilting and shrivelling of Clover is sometimes due to _Sclerotinia_, the mycelium of which pervades the roots and stock, on which the sclerotia may be found. Lucerne is similarly killed in Europe by the barren mycelium of _Leptosphaeria_, which may be found as a purple mat on the roots.

_Dwarfing_ consists in partial atrophy of all the organs, and is a common result of starvation in poor, dry, shallow soils, as may often be seen in the case of weeds on walls or in stony places. Dwarfs which are thus developed in consequence of perennial drought are not, however, necessarily diseased, in the more specific sense of the word; their organs are reduced in size proportionally throughout in adaptation to the conditions, and simply carry out their functions on a smaller scale.

Dwarfing is frequently a consequence of the lack of food materials, or of some particular ingredient in the soil, and in such cases is a diseased condition of some danger; similar results may ensue in soils containing the necessary chemical elements, but in unavailable forms.

Dwarfing may also be brought about by repeated maiming, nipping off the buds, pruning, etc., as in the miniature trees of the j.a.panese; and the case of trees continually browsed down by cattle, or of moor plants perennially dwarfed by cutting winds, are further ill.u.s.trations in the same category, as are also those of certain alpine and moraine plants, whose only chance of survival depends on their adapting themselves to the repeated prunings suffered by every young shoot which rises into the cutting winds, since there is no question of lack of food-materials in these cases.

The practice of the j.a.panese is to pinch out the growing tips of the shoots wherever they wish to prune back, and it is by the judicious use of this heading in, and suitable pot-culture, that the dwarfs are made, 6-20 inches high at from 30-80 years old.

Dwarfing is often brought about by grafting on a slow-growing stock, and this method is employed in practice, as are also heading in, pruning of roots, and confinement in pots.

Dwarfing may also be due to poor or shrivelled--partially atrophied--seeds or such as have had their endosperms or embryos injured by insects or fungi, and although it is possible to nurse such dwarfs into normal and vigorous plants with good culture, they do not usually recover under natural conditions in compet.i.tion with more vigorous plants.

_Distortions_ or _Malformations_ may be defined as abnormalities in the form of organs which concern all, or nearly all the parts, and do not refer merely to swellings or excrescences on them or excavations, etc., in them.

_Fasciation._--Shoots of Asparagus, Pine, Ash, and many other plants are occasionally expanded into broad ribbon-like structures often studded with more than the normal number of buds or leaves, etc., such as would be found on the usual cylindrical shoots. Such _fasciations_ are due to several buds fusing laterally under compression when young and the whole ma.s.s growing up in common, or, in a few cases, to the unilateral overgrowth of one side of the terminal bud. Fasciations appear to depend on excessive nutrition in rich soils. They may spread out above in a fan-like manner, exaggerating the abnormality, or they may revert to the original form. Some cases are more or less fixed by heredity--_e.g._ _Celosia_. Fasciated stems are frequently curved like a crozier, owing to one edge growing more rapidly than the other.

Cauliflowers are really cultivated monstrosities. Fasciated Dandelions, _Crepis_, monstrous Chrysanthemums, peloric _Linaria_, five-leaved Clovers, spiral Teazels, etc., may all, if grown with care, be kept more or less constant in the monstrous state. That is to say, the particular kinds of variation here manifested can be maintained in proportion as the external conditions controlling the variation are maintained. Such conditions are chiefly rich supplies of food-stuffs, plenty of water and air, suitable temperature and lighting, etc. Mutilations, favouring the development of abnormal buds may also induce fasciations.

_Torsions_ or spiral twistings of stems also frequently arise among plants grown in rich soils, and are often combined with fasciations--_e.g._ Asparagus, _Dipsacus_; and De Vries has shown that the peculiarity is not only transmissible by seed, but may be more or less fixed by appropriate culture.

_Contortions_ of stems are often due to the unequal growth on different sides of the stems owing to the presence of fungi--_e.g._ _Caeoma_ on Pines, _Aecidium_ on Nettles, also _Puccinia_ on petioles of Mallow, _Cystopus_ on inflorescences of _Capsella_, etc.

_Distortions_ of roots may be brought about in various ways by the hindrances afforded by stones.

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