Unrolling of Leaves of Musa sapientum and Some Related Plants and Their Reactions to Environmental Aridity

Abstract

The leaves of many spp. of Musaceae, Marantaceae, Zingiberaceae and Can-naceae are characterized by a well developed water-tissue, composed of large, thin-walled, hyaline cells which form one or more layers just beneath the upper epidermis. Observations and experiments on M. sapientum, Heliconia bihai, Calathea magnifica, Alpinia exaltata and Canna sp. show that the leaves, which in all these spp. are convolute in vernation, are forcibly unrolled through the agency of the cells of this water-tissue lying above and immediately adjacent to the principal transverse veins of the lamina, and which for this reason are termed "expansion cells." At the proper time these cells enlarge greatly, pushing the veins toward the upper (inner) surface, so that they are thrown into a state of compression, in which they cause the lamina to unroll. If the unrolling is prevented by binding, the expansion cells become several times their size in normally unfurled leaves. On the other hand, if the leaf is prematurely unrolled and held in a plane, the expansion cells fail to enlarge more than the neighboring cells of the water-tissue which take no part in unrolling. The leaves of Musa and Canna unroll normally in darkness. While the leaves of most related plants fold above the midrib in dry weather, thereby exposing the stomata, those of the banana fold below the midrib. The movements of the lamina halves are effected by special motor organs, the pulvinar bands, which lie along either side of the midrib throughout its length. The prismatic cells on the lower sides of these bands develop only when this side becomes concave, and in the light. Without them the pulvinar bands can not function.