上表皮
It is the stomata that makes the main difference between upper and lower epidermis of leaves. Animals possess skin as their outer most cover of the body. Similar to that, plants have a layer calledepidermisas their outermost cover. Epidermis originates from the protoderm. The outermost layer of the apical meristem and leaf primordium is called the protoderm. The whole plant body is covered by this single cell layered epidermis. Epidermis can be differentiated into upper and lower epidermis when it occurs on the upper and lower surfaces of a leaf. Therefore, upper (adaxial) surface and a lower (abaxial) surface of a leaf are called upper and lower epidermis respectively. Epidermal cells are barreled shaped and connected to each other in order to form the epidermis.
表皮展出的特殊功能是；Cutin层，后卫细胞，stomata, and trichomes. Both upper and lower epidermal cells secrete a waxy layer called cuticle. This layer helps to reduce theevaporationfrom leaves. The thickness of this layer varies according to the species and environmental conditions. Apart from that, leaf epidermis possesses several special cell types as guard cells and trichomes. The occurrence of these special structures varies on upper and lower epidermis.
The guard cells are bean or semi-lunar in shape (grasses consist of dumbbell shape guard cells). The minute pore surrounded by two guard cells is called a stoma. Unlike the epidermal cells, the guard cells havechloroplasts, thicker inner walls, and thin outer walls. They regulate the opening and closing of the stoma. Thus,transpirationis controlled by the guard cells. Furthermore, the guard cells are surrounded by two or more cells that are different from the usual epidermal cells called subsidiary cells. Stomata occurrence in the epidermis is varied in双子植物和单子叶植物.
What is Upper Epidermis?
上表皮包括barrel-shaped single cell layered epidermal cells. Epidermal cells in both monocots and dicots have the same shape and structure. Usually, the upper epidermis has a less number of guard cells compared to the lower epidermis. Some plants have stomata only on the upper epidermis; e.g. water lilies.
What is Lower Epidermis?
The Lower epidermis is similar to the upper epidermis in the formation and structure. However,occurrence of stomata and trichomes may differaccording to the species and environmental conditions. Stomata are abundant in the lower epidermis of a dorsiventral leaf. Dessert plants possess sunken stomata at their lower epidermis.
What is the difference between Upper and Lower Epidermis?
Both upper and lower epidermises are derived from the apical and leaf primordium. Both epidermal layers consist of a single layer of barrel-shaped cells. Epidermal cells are tightly linked to each other and provide mechanical strength and protection frompathogens和其他环境条件。叶表皮的壁由称为切蛋白的蜡质物质组成，可减少叶片的蒸发。一些植物由表皮警卫细胞周围的辅助细胞组成。
• Stomatal Density:
•上表皮的气孔密度比下表皮的密度少。
• Stomata Content:
• Floating plants consists of stomata only on the upper epidermis.
• Submerged plants do not have stomata on either epidermal layers.
•三植物的表皮：
• The Upper epidermis of xerophytic plants does not contain stomata.
• Lower epidermis of xerophytic plants consists sunken stomata.
Images Courtesy:
- The lower epidermis ofTilia×europaea通过Wikicommons（公共领域）
- 李尔的组织结构byZephyris(CC BY-SA 3.0)
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