LATICIFEROUS AND VESICULAR VESSELS, ETC. 



"3 



longer, and their septa become absorbed, according to Hanstein, by the growth of the 

 whole organ, by which the cells are extended. Thus long continuous tubes, filled 

 with raphides of enormous length, arise out of the rows of cells of the fundamental 

 tissue containing crystals. 



(b) The term Glands^ is applied to single cells or groups of cells which are 

 strikingly distinguished by their contents from the surrounding tissue, especially when 

 they contain odoriferous, strong tasting, coloured, oily, or resinous substances, which 

 find no further use in changes connected with nutrition or growth. Usually the cell- 

 walls also show certain differences from those of the adjoining cells, or they directly par- 

 ticipate in the formation of the cavity and of the secretion which it contains, they them- 

 selves becoming absorbed. A sharp boun- 

 dary-line can hardly be drawn, especially 

 between unicellular glands and single cells 

 with peculiar contents {e.g. tannin, crystals) 

 dispersed through the tissue. It is more 

 sharply marked in those that are compound ; 

 in them the mass of tissue which contains 

 the products of secretion is usually sur- 

 rounded by peculiarly developed layers, by 

 which the whole is clearly marked oflf and 

 individualised from the surrounding tissue ; 

 while generally the proper gland-tissue, 

 surrounded by it, is at length absorbed, 

 and forms a cavity filled by the products of 

 absorption of the cell-walls, and by the 

 coalescing cell-contents. The secretion 

 may collect in the interior of the gland 

 itself, as oil of camphor in single cells of 

 the parenchyma of the leaf of Camphora 

 officinarum, oil of citron in the cavities of 

 the large compound glands in the rind of the 

 fruit of species of Citrus ; or it may be dis- 

 charged externally, like the viscid excretion 



of the epidermis on the stem of Lychnis 'viscaria, the nectar of many nectaries, and 

 the blastocoUa of the viscid hairy covering of many leaf-buds {•vide infra). 



Gkmds may be classified according to their position as internal (/. e. lying in the 

 interior of the tissue), and superficial ; but doubtful instances occur. In both cases the 

 gland may consist of a single cell or a group of cells. Instances of internal simple glands 

 are the camphor-cells just mentioned, those of rhubarb containing chrysophane, the 

 gum-cells of Cactaceae, orchis tubers (salep), and the crystalliferous cells whose cavity 

 contains mucilaginous substances together with masses of crystals (sect. ii). Internal 

 compound glands are, on the other hand, those that contain essential oils in the rind 

 of the fruits of Citrus, as well as those covered only by the epidermis on the upper side 

 of the leaves of Dictamnm Fraxinella. The former are to be recognised, even in the 

 young ovary, as roundish groups of cells, the contents of which are distinguished by 

 turbid protoplasm and small drops of ©il ; the walls of these cells soon swell, then 

 become fluid, and thus form a spacious spherical space filled with mucilage and drops 

 of essential oil suspended in it. The layers of cells that surround the cavity form an 

 envelope, which marks it off sharply from the rest of the tissue. The formation of the 

 internal glands of Dictamnus (Fig. 96 c) commences with only two cells, one of which 

 belongs to the young epidermis, the other to the next layer of parenchyma ; the former 



^ [See also J. B. Martinet, Organes de secretion des v^getaux : Ann. des Sci. Nat. 5th ser. 

 vol. XIV.— Ed.] 



I 



Fig. 96.— Transverse section of the leaf of Psoralen hirta ; 

 e e epidermis ; / parenchyma containing chlorophyll ; m recep- 

 tacles of latex combining to form a gland. (After Hildebrand, 

 I.e.) 



