Fig. 40.—Large-coloured leaf occupying the position of the inflorescence in Gesnera, after Morren.
Fig. 41.—Ordinary arrangement of leaves in fascicles of three in Pinus pinea and unusual arrangement of leaves of same plant in spires.
Twisting of the stem is a frequent cause of the displacement of leaves (see spiral torsion), as also hypertrophy, whether that excess of development take place laterally or lengthwise (see elongation). Atrophy or suppression will also frequently bring about an alteration in the position of leaves; sometimes in such a manner that the place of the suppressed organ is occupied by another one. One of the most curious instances of displacement of leaves arising from suppression is that mentioned by Morren,[95] where, in Gesnera Geroltiana, a large leaf apparently occupied the extremity of the axis, a position which, under ordinary circumstances, no leaf could assume. The explanation given by the Belgian professor is, that the axis in this case, instead of throwing off a pair of leaves, one on each side, had from some cause or another produced only one; this one not only being much larger than ordinary, but brightly coloured, thus assuming some of the characteristics as well as the position of the inflorescence.
Alterations in the usual arrangement of leaves, however, are not always dependent on or coexistent with other teratological changes, but may simply depend on a natural elongation of the internodes, or on fission or multiplication; for instance, in some conifers, such as the Larch, (Abies Larix) or Pinus pinea, there may be found at different stages in the growth of the branches leaves in crowded fascicles or tufts; while, when growth is more rapid, the leaves may be disposed in a spiral or alternate manner.
In the yew (Taxus) the leaves at the ends of the shoots not unfrequently lose their usual distichous arrangement and become arranged in a close spiral manner, the elongation of the shoot being arrested. This appears to be the result of the injury effected by some insect.
