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 assume 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 unisexual (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 pricking the centre of the torus, and Molliard got double flowers by mechanical irritation.