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of mature plants, especially of those plants which have rosettes- 

 of leaves close to the ground. In such plants the short stem 

 increases slightly in length each year and the new leaves arise 

 higher and higher on the stem but, notwithstanding this, the 

 new leaves are always situated close to the ground. Trans- 

 verse wrinkles, or folds, at the base of the root, where the 

 tissues have shrunk, are often clearly visible where such 

 contraction has taken place. 



Structure of gl. The leaf is composed of funda- 



mental tissue traversed by vascular bundles and covered by 

 an epidermis, all of which are continuous with the corresponding 

 tissues of the stem. The xylem of each bundle is nearest the 

 upper surface of the leaf and the phloem nearest to the lower 

 surface. The fundamental tissue of the leaf may be uniform, 

 but is usually differentiated into, the so-called palisade tissue, 

 situated under the upper epidermis, and the spongy parenchyma, 

 which extends from the palisade tissue to the lower epidermis. 

 The palisade tissue consists of cylindrical cells, rich in chloro- 

 phyll, situated close together, with their long axes perpendicular 

 to the leaf surface. The spongy parenchyma consists of irre- 

 gularly shaped, often stellate, cells, containing less chlorophyll, 

 and between which there are large intercellular spaces. In the 

 leaf, as in other parts of the aerial shoot of the plant, the cells 

 of the epidermis fit close together except at the spots where the 

 openings, termed stomata, occur. These stomata are particularly 

 numerous on the leaves and in the leaves of land plants they 

 are found almost exclusively on the undersurface. A stoma 

 is formed by the division of an epidermal cell into two 

 daughter cells, which then separate from one another by the 

 splitting of the wall between them, an opening into the tissues 

 of the leaf thus being formed. These two cells contain 

 chlorophyll, are sausage-shaped, and are called the guard-cells. 

 Immediately below the guard-cells is a large intercellular 

 space which is in communication with the other intercellular 

 spaces of the leaf, so that the internal tissues of the leaf are 

 placed in communication with the air which can thus obtain 

 access to the cells, while the escape of gases and water vapour 

 from the leaf is also provided for. According to the quantity 

 of water in the guard-cells their free walls which adjoin the 

 stomatal opening, approach, or recede from, each other, thus 

 closing, or opening, the stoma. When the guard-cells are 

 turgid and full of water their free walls are drawn apart and the 

 stoma is opened, while if the guard-cells lose water and their 

 walls are no longer tightly stretched, as they are in turgescence. 

 their free walls are pushed together and close the stoma. 



