[208] 'Bot. Mag.,' tab. 5160, fig. 4. See also 'Gard. Chron.,' 1860, pp.

146, 170; 1861, p. 1092.

[209] 'Gard. Chron.,' 1851, p. 499.

CHAPTER V.

ALTERATIONS IN THE DIRECTION OF ORGANS.

The deviations from the ordinary direction of organs partake for the most part more of the nature of variations than of absolute malposition or displacement. It must also be borne in mind how frequently the direction of the leaves, or of the flower, varies according to the stage of development which it has arrived at, to unequal or disproportionate growth of some parts, or to the presence of some impediment either accidental or resulting from the natural growth of the plant. These and other causes tend to alter the direction of parts very materially.

=Change in the direction of axile organs, roots, stems, &c.=--The roots frequently exhibit good ill.u.s.trations of the effect of the causes above mentioned in altering the natural direction. The roots are put out of their course by meeting with any obstacle in their way. Almost the only exception to the rule in accordance with which roots descend under natural circ.u.mstances, is that furnished by _Trapa natans_, the roots of which in germination are directed upwards towards the surface of the water. So in _Sechium edule_, the seed of which germinates while still in the fruit, the roots are necessarily, owing to the inverted position of the embryo, directed upwards in the first instance.

A downward direction of the stem or branches occurs in many weak-stemmed plants growing upon rocks or walls, or in trees with very long slender branches as in _Salix Babylonica_, and the condition may often be produced artificially as in the weeping ash.

The opposite change occurs in what are termed fastigiate varieties, where the branches, in place of a.s.suming more or less of a horizontal direction, become erect and nearly parallel with the main stem as in the Lombardy poplar, which is supposed to be merely a form of the black Italian poplar.

M. de Selys-Longchamps has described a similar occurrence in another species of Poplar (_P. virginiana_ Desf.), and amongst a number of seedling plants fastigiate varieties may frequently be found, which may be perpetuated by cuttings or grafts, or sometimes even by seed; hence the origin of fastigiate varieties of elms, oaks, thorns, chestnuts, and other plants which may be met with in the nurseries.

Sometimes when the top of the main stem is destroyed by disease or accident, one of the heretofore lateral shoots takes its place, and continues the development of the tree in the original direction. It is often an object with the gardener to restore the symmetry of an injured tree so that its beauty may ultimately not be impaired.[210]

Climate appears sometimes to have some influence on the direction of branches, thus Dr. Falconer, as quoted by Darwin,[211] relates that in the hotter parts of India "the English Ribston-pippin apple, a Himalayan oak, a Prunus and a Pyrus all a.s.sume a fastigiate or pyramidal habit, and this fact is the more interesting as a Chinese tropical species of _Pyrus_ naturally has this habit of growth. Nevertheless many of the fastigiate varieties seen in gardens have originated in this country by variation of seeds or buds."

M. Carriere has also recorded a curious circ.u.mstance with reference to the fastigiate variety of the false acacia _Robinia pseudacacia_; he states that if a cutting or a graft be taken from the upper portion of the tree, the fastigiate habit will be reproduced, and the branches will be furrowed and covered with short p.r.i.c.kles; but if the plant be multiplied by detaching portions of the root-stock, then instead of getting a pyramidal tree with erect branches, a spreading bushy shrub is produced, with more or less horizontal, cylindrical branches, dest.i.tute of p.r.i.c.kles.[212]

=Eversion of the axis.=--In the case of the fig, the peculiar inflorescence is usually explained on the supposition that the termination of the axis becomes concave, during growth, bearing the true flowers in the hollow thus formed. The cavity in this case would probably be due not to any real process of excavation, but to a disproportionate growth of the outer as contrasted with the central parts of the fig. Some species of _Sempervivum_ have a similar mode of growth, so that ultimately a kind of tube is formed, lined by the leaves, the central and innermost being the youngest. The hip of the Rose may be explained in a similar manner by the greater proportionate growth of the outer as contrasted with the central portions of the apex of the flower-stalk. In cases of median prolification, already referred to, the process is reversed, the central portions then elongate into a shoot and no cavity is formed. A fig observed by Zuccarini (figs. 105, 106) appears to have been formed in a similar manner, the flower-bearing summit of the stalk not being contracted as usual, the flowers projected beyond the orifice of the fig. If this view be correct the case would be one rather of lengthening of the axis than of absolute eversion since it was never inverted.

[Ill.u.s.tration: FIG. 105.--Fig showing prolonged inflorescence and projecting flowers.]

[Ill.u.s.tration: FIG. 106.--Section of the same.]

=Altered direction of leaves.=--The leaves partake more or less of the altered direction of the axis, as in fastigiate elms, but this is not universally the case, for though the stem is bent downwards the leaves may be placed in the opposite direction; thus in some specimens of _Galium Aparine_ growing on the side of a cliff from which there had been a fall of chalk, the stems, owing apparently to the landslip, were pendent, but the leaves were abruptly bent upwards.

One of the most singular instances of an inverted direction of the leaves is that presented by a turnip (fig. 107) presented to the Museum of King's College, London, by the late Professor Edward Forbes. The turnip is hollow in the interior and the majority of the leaves springing from its apex instead of ascending into the light and air become bent downwards so as to occupy the cavity, and in such a manner as to bring to mind the position of an inverted embryo in a seed.

[Ill.u.s.tration: FIG. 107.--Hollow turnip, showing some of the leaves inverted and occupying the cavity.]

=Altered direction of the flower and its parts.=--The changes which take place in the relative position either of the flower as a whole or of its several parts during growth are well known, as also are the relations which some of these movements bear to the process of fertilisation, so that but little s.p.a.ce need here be given to the subject beyond what is necessary to point out the frequent changes of direction which necessarily accompany various deviations from the ordinary form and arrangement of parts.

In cases where an habitually irregular flower becomes regular, the change in form is frequently a.s.sociated with an alteration in direction both of the flower as a whole and, to a greater or less extent, of its individual members, for instance of _Gloxinia_, the normal flowers of which are irregular and pendent, there is now in common cultivation a peloriate race in which the flowers are regular in form and erect in position.

[Ill.u.s.tration: FIG. 108.--Flower of normal _Gloxinia_.]

[Ill.u.s.tration: FIG. 109.--Flower of _Gloxinia_, erect and regular (regular _Peloria_).]

Fig. 108 shows the usual irregular form of _Gloxinia_, with which may be contrasted figs. 109, 110 and 111.

Fig. 109 shows the regular erect form; fig. 110 the calyx of the same flower; while in fig. 111 are shown the stamens and style of the two plants respectively. In the upper figure the style of the peloriate variety is shown as nearly straight, and the stamens undergo a corresponding change. No doubt the relative fertility and capacity for impregnation of the two varieties is affected in proportion to the change of form. The Gloxinia affords an instance of regular congenital peloria in which the regularity of form and the erect direction are due to an arrest, not of growth, but of development, in consequence of which the changes that ordinarily ensue during the progress of the flower from its juvenile to its fully formed condition do not take place.

[Ill.u.s.tration: FIG. 110.--Calyx of erect _Gloxinia_.]

[Ill.u.s.tration: FIG. 111.--Stamens of erect regular, and of pendent irregular-flowered _Gloxinia_.]

A similar alteration accompanies this form of peloria in other flowers (see Peloria). A change in direction may result also from other circ.u.mstances than those just alluded to. Abortion or suppression of organs will induce such an alteration; thus in a flower of _Pelargonium_ now before me three of the five carpels, from some cause or other, are abortive and much smaller than usual, and the style and the beak-like torus are bent downwards towards the stunted carpels instead of being, as they usually are, straight.

Amongst orchids, where the pedicel of the flower or the ovary is normally twisted, so that the labellum occupies the anterior or inferior part of the flower, it frequently happens, in cases of peloria and other changes, that the primitive position is retained, the twist does not take place, and so with other resupinate flowers. In Azaleas a curious deflexion of the parts of the flower may occasionally be met with. Fig.

112 shows an instance of this in which the corolla, the stamens and the style were abruptly bent downwards: as young flowers of this singular variety have not been examined it is difficult to form an opinion as to the cause of this variation. In one plant the change occurred in connection with the suppression of all the flowers but one in the cl.u.s.ter, or rather the place of the flowers was occupied by an equal number of leafy shoots.

[Ill.u.s.tration: FIG. 112.--Flower of _Azalea_, showing the corolla reflected.]

Moquin[213] mentions a flower of _Rosa alpina_ in which two of the petals were erect, while the remaining ones were much larger and expanded horizontally. The same author quotes from M. Desmoulins the case of a species of _Orobanche_, in which a disjunction of the petals const.i.tuting the upper lip took place, thus liberating the style and allowing it to a.s.sume a vertical direction.

[Ill.u.s.tration: FIG. 113.--Flower of _Cuphea miniata_ enlarged, showing protrusion and hypertrophy of an erect placenta, after Morren.]

[Ill.u.s.tration: FIG. 114.--Placenta from the flower shown at fig. 113; the ovary is membranous and torn, the placenta, erect and ovuliferous, after Morren.]

M. Carriere[214] has described an instance wherein two apples were joined together, a larger and a smaller one; the former was directed away from the centre of the tree as usual, while the smaller one was pointed in exactly the opposite direction. The larger fruit had the customary parchment-like carpels, the smaller was dest.i.tute of them.

Sometimes the direction a.s.sumed by one flower as an abnormal occurrence is the same as that which is proper to an allied species or genus under natural circ.u.mstances; thus flowers of the vine (_Vitis_) have been met with in which the petals were spreading like a star (_fleurs avalidouires_), as in the genus _Cissus_.[215]

Morren describes a curious condition in some flowers of _Cuphea miniata_, in which the placenta protruded through an orifice in the ovary, and losing the horizontal direction became erect (figs. 113, 114). A similar occurrence happened in _Lobelia erinus_. To this condition the Belgian savant gave the name of gymnaxony.[216]

FOOTNOTES:

[210] The following details as to the method pursued by Mr. McNab, of the Edinburgh Botanic Garden, may not be uninteresting in this place.

They are from the pen of Mr. Anderson, and originally appeared in the 'Gardeners' Chronicle.'

"The mode of inducing leaders to proceed from laterals is a matter of comparatively little concern among the generality of deciduous trees, for they are often provided with subsidiary branches around the leader, at an angle of elevation scarcely less perpendicular, but the laterals of all Conifers stand, as nearly as possible, at right angles. Imagine the consternation of most people when the leader of, say, _Picea n.o.bilis_, _P. Nordmanniana_, or _P. Lowii_ is destroyed."

In a specimen of the latter plant the leader had been mischievously destroyed, to remedy which Mr. McNab adopted means which Mr. Anderson goes on to describe. "Looking from the leader downward to the first tier of laterals, there appeared to have been a number of advent.i.tious leaf-buds created, owing to the coronal bud being destroyed. These were allowed to plump up unmolested until the return of spring, when every one was scarified or rubbed off but the one nearest the extremity. To a.s.sist its development and restrain the action of the numerous laterals, every one was cut back in autumn, and this restraint upon the sap acted so favorably upon the incipient leader as to give it the strength and stamina of the original leader, so that nothing detrimental was evident twelve months after the accident had happened, and only a practical eye could detect that there had been any mishap at all. This beautifully simple process saved the baby tree.

"Another example of retrieving lost leaders may be quoted as ill.u.s.trative of many in similar circ.u.mstances. _Picea Webbiana_ had its leader completely destroyed down to the first tier of laterals. There was no such provision left for inducing leaf-buds as was the case with _P. Lowii_ above referred to. Resort must, therefore, be had to one of the best favoured laterals, but how is it to be coaxed from the horizontal position of a lateral to the perpendicular position of a leader? The uninitiated in these matters, and, in fact, practical gardeners generally, would at once reply, by supporting to a stake with the all-powerful Cuba or bast-matting. But no. A far simpler method than that, namely, by fore-shortening all the laterals of the upper tier but the one selected for a leader. Nature becomes the handmaid of art here; for without the slightest prop the lateral gradually raises itself erect, and takes the place of the lost leader. All that the operator requires to attend to is the amputation of the laterals until this advent.i.tious fellow has gained a supremacy. Singular provision in nature this, which, thanks to the undivided attention of a careful observer, has been fully appreciated and utilized."

[211] 'Variation of Animals and Plants,' ii, p. 277.

[212] Quoted in 'Gard. Chron.,' 1867, p. 654.

[213] Loc. cit., p. 315.

[214] 'Rev. Hortic.,' 1868, p. 110.

[215] Planchon and Mares, 'Ann. Sc. Nat.,' 5 ser., tom. vi, 1866, p.

228, tab. xii.

[216] 'Bull. Acad. Belg.,' xviii, part ii, p. 293.