. A textbook of botany for colleges and universities ... Botany. Fig. 53S. â Stem tip of Hippuris (a di- cotyledon), showing dermatogen (outermost layer), periblem (the five layers beneath the dermatogen), and the plerome (central region of more elongated cells). â After DeBaey. Epidermis.âThe dermatogen passes into the epidermis, which is usually a single layer of protective cells closely interlocked and with relatively impervious walls, but pierced by stomata. In case the stem increases in diameter, as in dicotyledonous trees, the epidermis is usually ruptured and destroyed, and another prot
. A textbook of botany for colleges and universities ... Botany. Fig. 53S. â Stem tip of Hippuris (a di- cotyledon), showing dermatogen (outermost layer), periblem (the five layers beneath the dermatogen), and the plerome (central region of more elongated cells). â After DeBaey. Epidermis.âThe dermatogen passes into the epidermis, which is usually a single layer of protective cells closely interlocked and with relatively impervious walls, but pierced by stomata. In case the stem increases in diameter, as in dicotyledonous trees, the epidermis is usually ruptured and destroyed, and another protective layer is developed by the cortex, as de- scribed below. Cortex. â The periblem pro- duces the cortex (figs. 539, 541), which is a hollow cylinder of living cells. It is exceedingly variable in structure, but is char- acterized by containing chloro- phyll tissue abutting against the epidermis; and if it is thick enough, there is also a deeper region of the cortex free from chlorophyll. The layer* of cortical cells abutting against the stele often forms a very distinct bounding layer, like an inner epidermis, and is called the endodermis (fig. 379). In the cortex, strands of fibrous cells may be de- veloped, and cavities or canals of â \'arious kinds may occur. In case the stem increases in diameter, the cortex develops a meristematic layer known as the phellogen or cork cambium (fig. 539), which forms cork cells, a very impervious kind of cell (see p. 318). Continuous activity of the phellogen within results in an increasing thick- ness of the sheet of cork cells with- out, and such sheets form a most efficient protection. The chloro- Fig. 539--Transverse section of outer phyll tissue beneath the cork cells portion of cortex of a geranium, showing maintains connection with the air ^'^^ ""'^ cambium or phellogen (c) cutting ,.,,., off layers of cork cells (four such layers for a time through special structures, beneath epidermis); below are chlorophyll, call
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