Vegetable Teratology / An Account of the Principal Deviations from the Usual Construction of Plants

by Maxwell T. Masters

CHAPTER II.
POLYMORPHY.

Usually the several organs of the same individual plant do not differ to any great extent one from another. One adult leaf has nearly the same appearance and dimensions as another; one flower resembles very closely another flower of the same age and so on. Nevertheless it occasionally happens that there is a very considerable difference in form in the same organs, not only at different times, but it may also be at the same time. Descriptive botanists recognise this occurrence in the case of leaves, and apply the epithet heterophyllous to plants possessed of these variable foliar characters. In the case of the flower, where similar diversity of form occasionally exists, the term dimorphism is used.

As these phenomena appear constantly in particular plants, they are hardly to be looked on, under such circumstances, as abnormal, but where they occur in plants not usually polymorphic, they may be considered as coming within the scope of teratology.

Heterophylly.—As a general rule, the leaves or leaf-organs in each portion of a plant, from the rhizome or underground axis, where it exists, to the carpellary leaf, have their own special configuration, subject only to slight variations, dependent upon age, conditions of growth, &c. The cotyledons are very uniform in shape in each plant, and are scarcely ever subject to variation. The leaves near the base of the stem, the root-leaves as they are not unfrequently called, sometimes differ in form from the stem-leaves; these again differ from the bracts or leaves in proximity to the flower. The floral envelopes themselves, as well as the bud-scales, all have their own allotted form in particular plants, a form by which they may, in most cases, be readily recognised. Hence, then, in the majority of plants there is naturally very considerable difference in the form of the leaf-organs, according to the place they occupy and the functions they have to fulfil; but, in addition to this, it not unfrequently happens that the leaf-organs in the same portion of the stem are subject to great variation in form. This is the condition to which the term heterophylly properly applies. The variation in form is usually dependent on a greater or less degree of lobing of the margin of the leaf; thus, in the yellow jasmine, almost every intermediate stage may be traced from an ovate entire leaf to one very deeply and irregularly stalked. Broussonettia papyrifera, and Laurus Sassafras, and the species of Panax, may be mentioned as presenting this condition. Sometimes in the last-named genus, as also in Pteridophyllum, every gradation between simple and compound leaves may be traced. The horse-radish (Cochlearia Armoracia) may also be instanced as a common illustration of polymorphism in the leaves. In ferns it is likewise of frequent occurrence, markedly so in Scolopendrium D'Urvillei, in which plant every gradation from a simple oblong frond to an exceedingly divided one may be found springing from the same rhizome at the same time.

Fig. 177.—Syringa persica laciniata, showing polymorphous leaves.

A similar protean state, but little less remarkable, occurs in many of our British ferns, notably in Scolopendrium vulgare, of which Mr. Moore enumerates no fewer than 155 varieties,[366] many of the forms occurring on the same plant at the same time. Cultivators have availed themselves of this tendency to produce multiform foliage, not only for the purposes of decoration or curiosity, as in the many cut-leaved or crisped-leaved varieties, but also for more material uses, as, for instance, the many varieties of cabbages, of lettuces, &c. Most of these variations are mentioned under the head of the particular morphological change of which they are illustrations.

The effect of a change in the conditions of growth in producing diversity in the form of the leaf may be here alluded to. Ficus stipulata, a plant used to cover the walls of plant-stoves in this country, and growing naturally on walls in India, like ivy, produces leaves of very different form, size, and texture, when grown as a standard, from what it does when adhering to a wall. Marcgraavia umbellata furnishes another example of a similar nature, as indeed, to a less extent, does the common ivy.

Allusion has been already made to the occasional persistence of forms in adult life, which are commonly confined to a young state, as in the case of some conifers which present on the same plant, at the same time, two different forms of leaves. Mention has also been made of the presence of adventitious buds on leaves and in other situations. The leaves that spring from these buds are usually of the same form as the other leaves of the plant, but now and then they differ. Of this a remarkable illustration is afforded by a fern, Pteris quadriaurita, in which the fronds emerging from an adventitious bud are very different from the ordinary fronds.