In the case of herbaceous plants, in the case of plants like most of the ferns where the stem is on the underground rootstock (Pteris), or a very short erect stem, as in the Christmas fern, the leaf replaces the aerial stem, and the division (or branching, as it is sometimes styled) of the leaf corresponds to the branching of the stem. This is more marked in the gigantic exotics like Cibotium regale, and in the tree ferns which have quite tall trunks, the massive compound leaves replace branches. In the palms and cycads are similar examples. Those who choose to observe can doubtless find many examples close at hand. (3) While divided leaves have probably not been evolved in response to the light relation, still their relation in this respect is an important one, since if the leaf with its present size were entire, it would cast too dense a shade on other leaves below. =760. General structure of the leaf.=—The general structure of the leaf has been already studied (see Chapters IV, V, VII). It is only necessary to recall the main points. The upper and lower surfaces of the leaf are provided with a layer of cells usually devoid of chlorophyll. The mesophyll of the leaf consists usually of a layer of palisade cells beneath the epidermis, and the remainder consists of loose parenchyma with large intercellular spaces. Through the mesophyll course the “veins,” or fibrovascular strands, consisting of the xylem and phloem portions and serving as conduits for water, salts, and foodstuffs. In the epidermis are the stomata, each one protected by the two guard cells. The guard cells as well as the mesophyll contain chlorophyll. The stomata and the communicating intercellular spaces furnish the avenues for the ingress and egress of gases, and for the escape of water vapor. =761. Protection of leaves.=—There are many modifications of the general plan of structure in different leaves, many of them being adaptations for the protection of the leaf under adverse or trying conditions. Many leaves are also capable of assuming certain positions which afford them protection. The discussion of this subject may be presented under two general heads: Protective modifications; protective positions. II. Protective Modification of Leaves.