Anatomy of the Cotton Leaf and Effects Induced by 2,4-Dichlorophenoxyacetic Acid

Abstract

The marginal histogen of the cotton leaf produces 3 internal layers of plate meristem. The adaxial of the 3 layers becomes the palisade layer of the mature leaf. Divisions producing cells vertical to the major plane of the leaf lamina increase the 2 abaxial layers to 3 layers that become spongy mesophyll. Minor veins are initiated by divisions in a row of cells in the adaxial of the spongy mesophyll layers. The first division of one of these cells produces cells in a plane vertical to the plane of the plate meristem. Adjacent cells in the same and adjacent layers become involved in vein formation. The black, hollow spheres in vein parenchyma and in mesophyll originate with the enlargement of single cells. Each cell becomes a group of cells with darker-staining cytoplasm. The group of cells develops into a black, hollow sphere with a hyaline central cavity. Treatment of the expanded laminae of the cotyledons of young plants with approx. 40 mg. of a 0.2% concn. of 2,4-dichlorophenoxyacetic acid in lanolin inhibited activity of the marginal histogen and of the plate meristem in subsequently formed leaves. Major veins become fasciated. Chlorenchymatous tissue developed only in islets in the central portion of each leaf and along the leaf margin.