The morphological structure and physiology of leaves are developed from leaf primordium. Generally speaking, leaves (complete leaves) consist of three parts: leaves (leaves or veins), petioles and stipules. The leaves are flat and green and can be used for photosynthesis. Petiole is the structural part connecting stem and leaf, and it is the channel of material transportation between stem and leaf, which can adjust the position and direction of leaf. Stipules are a pair of appendages at the base of petiole, which have various shapes, such as green flake (photosynthesis), spiny (protection), sheath phimosis (protection of axillary buds) and so on. Among them, leaves are composed of epidermis, mesophyll and veins, and their structural characteristics play different roles in plant physiology. Epidermal cells are closely arranged and transparent, and there is cuticle outside, which plays a role in protecting leaves. The epidermis has pores, which are "windows" for gas exchange. Plants can absorb carbon dioxide in the air through stomata and provide raw materials for photosynthesis. Mesophyll cells are located between the upper and lower epidermis, and they contain a large number of chloroplasts, which are the main part of plant photosynthesis. The mesophyll cells of most plants differentiate into palisade tissue and sponge tissue. The palisade tissue is close to the epidermis and consists of some closely arranged long columnar cells, which are mainly used for photosynthesis. Sponge tissue, close to the lower epidermis, is composed of some loosely arranged irregular cells with well-developed intercellular spaces, which are mainly used for gas exchange and photosynthesis. There are conduits and sieve tubes in the veins, which transport water, inorganic salts and organic matter, and also contain mechanical tissues for support. Leaf is the organ with the largest surface area exposed to air, so the external environment has obvious influence on the morphological structure of leaf. In the process of evolution, plants adapt to different ecological environments and form leaves of various ecological types. According to the relationship between plants and water, plants can be divided into xerophytes, mesophytes and aquatic plants. The leaves of xerophytes have many adaptive characteristics in morphological structure, but the same characteristic is that the ratio of leaf surface area to volume is relatively low, that is, the evaporation surface of leaves with the same volume is small. The characteristics of dry leaves mentioned above, such as small leaf shape, close arrangement of fleshy leaves and mesophyll, developed palisade tissue and transport tissue, many stomata and subsidence, are all related to reducing the evaporation area of leaves. According to the water depth of the growing environment, aquatic plants are divided into whole submerged plants immersed in water; Floating plants with leaves floating on the water; There are three kinds of emergent plants, most of whose stems and leaves protrude out of the water, and their roots grow in the water. In the process of long-term adaptation to aquatic environment, aquatic plants have formed special structures in their bodies, and the changes of their leaf structures are particularly significant. For emergent plants, the leaf structure is similar to that of ordinary mesophytes except that the intercellular space is developed or sponge tissue accounts for a large proportion. Submerged plants have thin leaves, undeveloped mesophyll tissue, no palisade tissue and sponge tissue differentiation, and mesophyll is all composed of sea surface tissue. There are many chloroplasts in mesophyll cells, and mesophyll cells are developed, forming developed aeration tissue. There are few veins, underdeveloped or even degraded xylem, no horny membrane or thin epidermis and no stomata. Epidermal cells have chloroplasts and can carry out photosynthesis. The upper and lower surfaces of floating plant leaves develop in both xerophytic and aquatic directions. Epithelial cells have thick cuticle and waxy layer, stomata are all distributed in epidermis, and there are several layers of palisade tissue closely arranged near the upper epidermis, and mesophyll contains mechanical tissue. There are large intercellular spaces between mesophyll cells near the lower epidermis, forming well-developed ventilation tissue, and the cuticle of the lower epidermis cells is thin or not. Light intensity is another important factor affecting the structure of leaves. Photosynthesis of many plants is suitable for strong light, but it can't stand shade. This plant is called sunflower. Most crops, including rice, fall into this category. Another kind of plants, whose photosynthesis is suitable for low light and its photosynthetic efficiency is low under full light, are called shade plants. Many undergrowth plants fall into this category. Leaf is an organ that directly receives light, so its morphological structure is also greatly influenced by light. The leaves of sunny plants are called solar leaves, which often have the structure of dry leaves because of their strong light and heat. Leaves are generally smaller and thicker. The epidermis is covered with a thick stratum corneum; Some leaves are densely covered with fluff or silvery white scales, which can reflect strong light; Stomatal apparatus is small and dense, often sinking; The mesophyll cells are small, closely arranged, the leaves are light in color, the sponge tissue is underdeveloped, and the palisade tissue is developed, often with 2 ~ 3 layers, and sometimes the upper and lower epidermis of the leaves have palisade tissue. The mechanical structure is also very developed, and the veins are long and thin. The leaves of shade plants are called shade leaves, because plants are in the shade for a long time, and their structures tend to those of aquatic plants. The vulva leaves are large and thin, and the palisade tissue is underdeveloped; The intercellular space is developed, chloroplasts are large, leaves are dark green, epidermal cells often have chloroplasts, stomata are few, and the cuticle of epidermal cells is thin. These characteristics are suitable for absorbing and using scattered light for photosynthesis under shading conditions. The leaves of sun plants are often arranged at a certain angle with direct light, and the mosaic of leaves is not obvious. However, the petiole of negative plants is long or short, and the leaf shape is large or small, so that the leaves are arranged in a mosaic shape on the same plane to make use of insufficient sunlight. The morphological structure of leaves in different light-receiving parts of the same plant will also obviously show the properties of positive leaves and negative leaves. Leaves near the top and facing the sun tend to be positive, while leaves in the shade tend to be negative. The flag leaves of rice generally have high photosynthetic intensity, and one of the internal reasons is that they have the structural characteristics of solar leaves. Therefore, it is very important to prevent premature senescence of leaves, make photosynthesis more effective, make young panicles supply photosynthetic products continuously, realize grain fullness, and ensure the sustainable growth of flag leaves and upper second and third leaves. In a word, in order to adapt to different ecological environments, the leaf morphological structure of plants has changed accordingly, so that plants can survive better.