Part 18 (1/2)
_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.