Fig. 2—Cohesion of two branches in Dipsacus sylvestris.
The first of these is most commonly met with, doubtless owing to the number of the branches and the facilities for their union. An illustration of it is afforded by the figure (fig. 2), showing cohesion affecting the branches of a teazle (Dipsacus sylvestris). Union of the branches may be the result of an original cohesion of the buds, while in other cases the fusion does not take place until after development has proceeded to some extent. Of this latter kind illustrations are common where the branches are in close approximation; if the bark be removed by friction the two surfaces are very likely to become united (natural grafting). Such a union of the branches is very common in the ivy, the elder, the beech, and other plants. It may take place in various directions, lengthwise, obliquely, or transversely, according to circumstances. This mode of union belongs, perhaps, rather to the domain of pathology than of teratology. Some of the instances that have been recorded of very large trees, such as the chestnut of Mount Ætna, are really cases where fusion has taken place between several of the branches, or suckers, thrown out from the same original stem.[10] The same process of grafting occurs sometimes in the roots, as in Taxus baccata mentioned by Moquin, and also in the aerial roots of many of the tropical climbing plants, such as Clusia rosea, &c.
Fig. 3.—Fasciation in Lettuce.
Fasciation.—In the preceding instances of union between the branches, &c., the actual number of the fused parts is not increased; but if it happen that an unusual number of buds be formed in close apposition, so that they are liable to be compressed during their growth, union is very likely to take place, the more so from the softness of the young tissues. In this way it is probable that what is termed fasciation is brought about. This is one of the most common of all malformations, and seems to affect certain plants more frequently than others. In its simplest form it consists of a flat, ribbon-like expansion of the stem or branch; cylindrical below, the branches gradually lose their pristine form, and assume the flattened condition.
