524 PHYSIOLOGY. 



ever different in appearance, always forming and storing or con- 

 veying hydro-carbonaceous secretions. 



Glands are the structures of this kind most frequently met with, 

 and they are generally connected in some manner with the epider- 

 mal tissue. Glands may be divided into simple and compound, 

 and also into external and internal. 



Simple external glands are in most cases glandular hairs ; i. e. the ter- 

 minal cell (or cells) of a jointed hair is expanded or filled with oil or 

 other secretion. Of this nature are the glands of the foliage, flowers, 

 &c. of many Labiatae, Scrophulariaceae (fig. 567, /), &c. 



Simple internal glands are mostly isolated cells of the layer imme- 

 diately subjacent to the epidermis, as in the leaves of Beyoma, Lysi- 

 machia vulgaris, the petals of Mac/nolia, &c. Such glands occur also in 

 the leaves of Lauraceae. The cystolithes of Urticaceae are related to 

 these (p. 506). 



Compound external glands are sometimes hair-like growths from the 

 epidermis, or from the deeper tissues, from whicji they form outgrowths 

 (Droserd). The summit or the base (Dictamnus, fig. 567, a) is sometimes 

 developed into a cellular nodule, the cells of which either contain the 

 secretion or surround a large central cell filled with it. Other superficial 

 glands form papillae of various shapes, in like manner either wholly 

 formed of secreting cells, or with a central reservoir, as in the Hop, 

 Begoniaceae, Rosaceae, Leguminosae, &c. 



Compound internal glands are commonly reser- Fig. 572. 



voirs surrounded by a special layer of cells, lying 

 just beneath or sometimes rising in a dome shape 

 a little above the surface of the epidermis. Ex- 

 amples of this occur in the leaves of Ruta (fig. 

 572), rind of the fruit of Oranges, Lemons, c., 

 leaves and stems of Hypericaceae, Myrtaceae, &c. 



None of these glands have excretory ducts like 

 the "lands of animals. In many cases the secre- 

 tions exude through the membrane, and give a 

 peculiar character to the surface of the organs in O f R U t a 

 which they are found. A very general form of Magn. 50 diam, 

 secretion of this kind is the exudation of saccharine 



fluid from the superficial cells, very common at the base of petals and 

 ovaries, on the stigma, and sometimes on leaves or at particular points of 

 the lower surface of the leaves, as of Prunus Laurocerasus, the^ Laurus- 

 tinus, and other shrubs. (For fuller information on these subjects, the 

 memoirs of Trecul, Van Tieghem, and Martinet should be consulted.) 



Stings are a form of glands, consisting of a long, stiff and pointed 

 hair expanded into a bulb at the base containing the poison. This 

 bulb is surrounded by a layer of cells derived from the epidermis, 

 which by their tension exert a certain pressure, whence ifc results that 

 when the point of the stinging-hair is broken off, the fluid is pressed 

 out from the orifice. 



Reservoirs for peculiar secretions may b$ regarded as a highly 

 developed form of the internal glands. They consist of tubes 



