520 



PHYSIOLOGY. 



dermal products in which the cells become thickened by woody 

 secondary deposits. 



Fig. 567. 



Epidermal appendages : a, gland of Fraxinella, in vertical section ; b, simple jointed 

 hair of Pelargonium ; c, hair of Sisymbrium Sophia ; d, hair of garden Chrysan- 

 themum; e, hair of a Grevillea; f, hair of the bulbil of Achimenes, with a glan- 

 dular terminal cell. All magn. 50 diam. 



Thickening layers of Epiderm-cells. The most remarkable 

 diversities of condition of texture of herbaceous organs depend on 

 the consistence which the epidermal layer acquires. The leathery- 

 texture of evergreens, the woody character of the leaves of Coni- 

 fers, &c. depend chiefly on thickening of the wall of the epidermal 

 cells. 



In all epidermis exposed to the air, the outer walls of the cells 



Fig. 568. 



Fig. 569. 



Fig. 568. Vertical section of epidermal cells of the leaf of Hoy a carnosa : a, the portion of the 

 secondary layer coloured yellow by iodine. Maga. 440 diam. 



Fig. 569. Section as in fig. 568, treated with caustic potash : a, detached cuticular pellicle ; b, 

 the layers of thickening of the outer walls of the cells. Magn. 440 diam. 



become early strengthened by secondary thickening; these are 

 very thin and slight in soft herbaceous leaves, especially when such 

 plants are reared in a warm, moist atmosphere. In leathery or 

 hard leaves, also in the thick tough leaves of succulent plants, 

 such as the Aloes, Hoya (figs. 568 & 569), &c., the secondary 

 layers acquire great thickness ; and in the epidermis of the 

 branches of Viscum (fig. 570) the cells become absolutely filled up, 

 and the cells of the subjacent layer of tissue also suffer the 

 same change. 

 In the course of this thickening, the superficial laminae, exposed to"_the 



