228 A TEXT-BOOK OF BOTANY. 



latter in some cases being lined by a papillose epithelium or a 

 form of bracket-cells, etc. They are found in the following fami- 

 lies: Araliaceae, Bixaceae, Caesalpinaceae, Compositae, Connaracese 

 (with sphaero-crystalline contents), Euphorbiaceae (with bracket- 

 epithelium), Geraniaceae (with sphaero-crystalline contents), 

 Guttiferae, Leguminosae (intramural glands with a papillose epi- 

 thelium or bracket-epithelium), Lythraceae, Malpighiaceae, Mal- 

 vaceae, Meliaceae, Menispermaceae, Myrtaceae, Oleaceae, Passi- 

 floriaceae, Piperaceae, Podostemaceae, Polygalaceae, Polygonaceae 

 (secretory cavities sometimes formed from four epidermal cells), 

 Primulaceae (occasionally with red crystalline contents), Pro- 

 teaceae (intramural glands), Rhamnaceae (with a papillose epi- 

 thelium), Rosaceae, Rubiaceae, Rutaceae, Simarubaceae, Styracaceae, 

 and Theaceae. 



3. Secretory canals differ from secretory cavities in that they 

 are more or less elongated receptacles and often referred to as 

 oil-ducts or oil-tubes. Like the secretory canals, they originate 

 variously and have diverse contents. They may occur in a 

 number of different portions of a plant, but their distribution is 

 quite characteristic of certain genera or even of families. Secre- 

 tory canals have been observed in the following families : Ana- 

 cardiaceae, Araliaceae, Burseraceae, Cactaceae, Caesalpinaceae, 

 Celastraceae, Compositae, Gesneraceae, Guttiferae, Hamamelidaceae, 

 Leguminosae, Pittosporaceae, Podostemaceae, Rhamnaceae, Ruta- 

 ceae, Simarubacese, Theaceae, and Umbelliferae. (Consult Sole- 

 reder's "Systematic Anatomy of the Dicotyledons.") 



4. Glandular hairs. Volatile oils and resins arise in the 

 glandular hairs formed on the surface of stems, leaves, and 

 various parts of the flower in the Labiatae, Compositae, and other 

 families. In these hairs a volatile oil separates in the form of 

 large, oily globules which lie between the cuticle and the outer 

 wal] of the underlying cells (Fig. 124). The origin of this secre- 

 tion has been variously ascribed to the protoplasmic content of the 

 cell or to a modification of the cell-wall. In the former case it is 

 said to arise as a metabolic substance in the protoplasm, and is 

 later diffused into the glandular area between the outer cellulose 

 wall and cuticle. While this manner of formation of the oily 

 secretion would seem reasonable, yet the studies by Tschirch 



