July 1891.] BOTANICAL SOCIETY OF EDINBURGH. 187 



and more fleshy consistence, the cells having become filled 

 with refractive material. They persist, and enlarge but 

 little. At older nodes they are more deeply coloured, shiny, 

 and resinous, and bear some resemblance to scale-insects. 

 Stomata occur on them in considerable numbers. A trans- 

 verse section shows that the epidermal cells of the upper 

 (inner) surface form a columnar layer, most distinctly difteren- 

 tiated where it is interrupted by the pedicel of the stipule. 

 The bounding membrane is very delicate, and the cells are 

 commonly filled with homogeneous brown material which 

 here, as in the leaves of dried specimens, resists the action of 

 potash and alcohol. The lower (outer) epidermis is not 

 columnar, and has a well-marked cuticle. To all appear- 

 ance the inner surface is most actively secretory. The 

 stipules separate laterally in the course of gi'owth of the 

 node. Tor a time the space between them is green and 

 non-resinous, but ultimately and very commonly the entire 

 node becomes the seat of resinous exudation. The stipules, 

 which at first stand out conspicuously as dark brown scales, 

 are gradually lost sight of in the resin. In branches -^ inch 

 in diameter, the largest available for examination, the nodes 

 were swollen and encased in a thick zone of brittle resin. 

 The area of exudation, however, usually becomes located on the 

 sunny side, leaving a portion of the opposite side bare. 



The excretions of the leaves and stipules will serve to 

 protect the growing parts against drought.* 



The exuded material has been turned to account com- 

 mercially. It (and also an excretion of Acacia G~reggii, Gray, 

 it is said) yields the product known as Arizona shellac or 

 Sonora gum. The following statement is made by American 



authorities:! — "These lac-yielding plants are as 



plentiful as the so-called sage-bush from Southern Utah to 

 New Mexico, and from the Colorado desert to Western 

 Texas, lac being most abundant round stations on the 

 Mojave and Colorado deserts. The exudations which take 

 place as the result of an insect's sting can be easily collected 

 by boiling the twigs in water, the gum (?) which rises to the 

 top being scummed off, strained and dried on smooth stones, 

 and hand pressed into flakes ready to make sealing-wax or 



* Pringle, in Garden and Forest, vol. i. p. 524. 



t Stillraann and Redding, vide Pharm. Jour., 3rd ser., vol. x. p. 962. 



