INTKRCELLULAK AND EPIDERMAL SYSTEMS. 



51 



ranged, and are as elaborately constructed as any proper organ can 

 be. They are built up of cells in a manner ■which may be compared 

 to a stack of flues or chimneys built of brick : they are constructed 

 upon a uniform plan in each species, and are evidently essential 

 parts ; plants which grow in water requix'ing a full supply of air 

 in their interior. Fig. 68 shows some of these air-passages in the 

 flowex'-stalk of Calla ^thiopica. 



67. Receptacles of Special Secretions. These arise from the exuda- 

 tion of the proper juices of the cells into intercellular passages, 

 which are distended by the accumulation ; or from the obhteration 

 of contiguous cells, so as to form cavities of considerable size. Such 

 are the turpentine-canals of the Pines, «&:c. ; the oil-cells of the 

 fruit of the Umbelliferae, and those in the rind of the orange and 

 lemon ; the latex-canals in Sumach, &c. Liternal Glands, such as 

 those which form the translucent dots in the leaves of the Orange 

 and Myrtle, are little clusters of cells, filled with essential oil. 



68. Epidermal System. In most plants, except of the lowest grades, 

 and those which grow under water, the superficial layer of cells is 

 different from the rest, and forms 



69. The Epidermis, or skin of the plant. This consists of one or 

 more layers of empty thick -walled cells, cohering so as to form a 

 firm and close membrane, wliich may be detached from the subjacent 

 tissue. It covers all parts of the pkmt which are directly exposed to 

 tlie air, except the stigma. Its structure and office will be described 

 in the chapter on the Leaves. The epidermis forais a complete 

 and continuous covering, except that in certain parts, especially on 

 the lower surface of the leaves, it is perforated by a multitude of 

 small openings, called 



FIG. 68. A magnified slice across part of the flower-stalk of Calla Jithiopica of our green- 

 houses, showing the large air-passages, built up of cells j nearly in the centre, a bundle of 

 woody tissue is seen "in cross-section. 



