Anatomy of the Aerial Epidermis of the Mistletoe, Phoradendron flavescens

Abstract

The epidermis of the American mistletoe, Phoradendron flavescens, was studied in stems and leaves in several stages of development. Young plant parts have numerous trichomes. The trichomes are commonly unicellular and occur singly or in groups of two or more. Some trichomes deposit a secondary wall that subsequently becomes lignified. As trichomes age, the lumen in the part of the cell extending above the surface becomes isolated from the lower part. Constriction and/or partition formation are involved in separating the lumen into distinct regions. Ultimately, the aerial extension is lost, but the lower part, which contains the nucleus, remains as an integral part of the epidermis. As stems (and leaves) develop, their cuticular covering becomes very thick and is heterogeneous in appearance. The thick covering restricts tangential expansion of epidermal cells; as a result, breaks occur along the axis of the stem. Where the breaks occur, a new cuticular layer is produced by cells in a subepidermal position. In many instances, the subepidermal cuticular layer is within the confines of the cell (but outside a thin cellulose layer) rather than outside the outer periclinal wall. As a consequence, the cuticular layer in a cell is separated from that in adjacent cells by the anticlinal walls between the cells. Lateral contiguity is established when, during development, the anticlinal walls become detached from the inner periclinal wall near their points of juncture. Other changes concomitant with the development of a thick cuticular layer include the compression of some stomates and the pulling apart of others, the formation of a cuticular layer by subepidermal cells other than those associated with breaks, the death of some epidermal cells, and the general distortion of the surface layer near nodes and in conjunction with grooves that form perpendicular to the stem axis. The complex layer so developed is termed the cuticular epithelium. In the case of P. flavescens the cuticular epithelium is the ultimate protective covering of the stem.