Tissues

At the tissue level the leaf blades have certain differences from those of other plants, but the same general picture prevails. There are an upper and a lower epidermis, the latter with many stomata (microscopic pores). In between, the mesophyll is usually composed of cells with large intercellular spaces. In thicker leaves the upper mesophyll is composed of palisade cells—elongated cells arranged parallel and oriented with the long axis vertical to the leaf surface. The veins range from the massive major midribs, or rachises, which have well-defined xylem, phloem, pericycle, and endodermis, to the delicate capillaries represented by little more than a single file of tracheids that sometimes ends in a cluster of somewhat modified tracheids. Underneath a sporangium or sorus, the veins may become dilated and multilayered (so-called fertile veins). In many ferns all or nearly all of the photosynthesis is accomplished by the epidermis, the mesophyll having been eliminated in evolution. An example is the common maidenhair fern (Adiantum pedatum), the blade of which, between veins, is mainly made up of only two layers, the upper and the lower epidermis, in which most photosynthesis occurs.

The indument of fern leaves may be like that of the stem, but usually the trichomes (plant hairs) or scales are fewer, more widely separated, and smaller, or they may be of a different type. Numerous ferns are described as having glabrous (bald) leaf blades, but many of these actually have at least a few microscopic trichomes, which are usually glandular and appressed to the blade surface.