It will be seen that under this general head of hypertrophy, increase of size, however brought about, is included; thus, not only increase in length, but also in thickness; alterations of substance or consistence, no less than of dimensions, are here grouped together. The alterations of consistence resulting from an inordinate development of cellular, fibrous, or ligneous tissue, are, of course, strictly homologous with the similar changes which occur, under ordinary circumstances, during the ripening of fruits or otherwise.
Hypertrophy, whatever form it may assume, may be so slight as not perceptibly to interfere with the functions of the part affected, or it may exist to such an extent as to impair the due exercise of its office. It may affect any or all parts of the plant, and is generally coexistent with, if not actually dependent on, some other malformation. Thus, the inordinate growth of some parts is most generally attended by deficiency in the size and number of others, as in the peripheral florets of Viburnum or Hydrangea, where the corollas are relatively very large, and the stamens and pistils abortive.
CHAPTER I.
ENLARGEMENT.
A swollen or thickened condition (renflement) is usually the result of a disproportionate formation of the cellular tissue as contrasted with the woody framework of the plant. We see marked instances of it in cultivated carrots and turnips, the normal condition of the roots or root-stocks in these plants being one of considerable hardness and toughness, and their form slender, tapering, and more or less branched.
The disproportionate development of cellular tissue is also seen in tubers and bulbs, and in the swollen stems of such plants as Echinocactus, Adenium obesum, some species of Vitis, &c. So, too, the upper portion of the flower-stalk occasionally becomes much dilated, so as ultimately to form a portion of the fruit. But it is not necessary to give farther illustrations of this common tendency in some organs to become hypertrophied. As a result of injury from insects or fungi, galls and excrescences of various kinds are very common, but their consideration lies beyond the scope of the present work.
Fig. 200.—Pelargonium, one branch of which was hypertrophied.
Enlargement of axile organs.—All the species of Pelargonium, Geranium, Mirabilis, as well as those of Caryophylleæ and other orders, have tumid nodes as a normal occurrence. In the genus Pelargonium this swelling is sometimes not confined to the nodes, but extends to the interspaces between them, e.g. P. spinosum. This condition, which happens as a natural feature in the species just named, may also occur as an exceptional thing in others. The author is indebted to Dr. Sankey for a branch of Pelargonium which was thus thickened, the remaining branches not being in any way affected. The leaves on the swollen branch were smaller than the others, and their stalks more flattened. There was, in this instance, no trace of fungus or insect to account for the swelling of a single branch, which might, therefore, be due to bud-variation, perhaps to reversion to some ancestral form. The repeated cross fertilisations to which Pelargoniums have been subjected render this hypothesis not an improbable one.
As an accompaniment to a spiral torsion of the woody fibres, this distension of the stem is frequently met with, as in Valeriana, Dipsacus, &c. (See Spiral Torsion.)