Verjetahk Physiology. 41 1 



filled with the protoplasm or albuminous substance, and that the 

 oil is excreted in drops into hollows formed in this. As the oil- 

 drops increase, and the protoplasm is absorbed or consumed, the 

 drops coalesce, until at leng-th they displace all the protoplasm 

 except the layer lining- the wall of the cell. The conditions are 

 analogous to those above described of the cells containing watery 

 fluid colouring matters ; and it is through the agency of the thin 

 layer of albuminous matter that the oil is prevented from exuding 

 through the cell-wall in the living cell. When this pellicle is 

 disorganised the oil soaks readily through the cellulose-membrane. 

 Most of these aromatic oils consist of a mixture of an oil liquid 

 at common temperatures with a substance analogous to camphor 

 or stearoptene, solid at common temperatures when separate, 

 Stearoptenes are found alone, generally in a crystalline form, in 

 the wood of various aromatic trees ; but their developmental 

 history, as well as that of the various resins, is still involved 

 in obscurity. 



The so-called " milky juices," such as abound in the roots of 

 lettuces, dandelions, the stem and foliage of poppies, spurges, 

 <&c., exist in special tubular reservoirs, which appear to be inter- 

 cellular passages into which the secretion has exuded ; some 

 authors, however, believe these canals to be formed by the con- 

 fluence of rows of cells. The juices are not " milky " until ex- 

 posed to the air ; they consist of clear fluid containing dissolved 

 albuminous matters, with globules of resin and sometimes starch- 

 granules in suspension ; the milkiness disappears again when 

 they dry up, and there remains a resinous substance, varying in 

 consistence and composition in different plants — often, as in the 

 case of the opium-poppy, the lettuce, &c., containing the active 

 principles to which the plants owe their economic or medicinal 

 value. 



Nothing general can be stated at present regarding the pro- 

 duction of these juices, which, however, deserve careful investi- 

 gation, since they exist in small quantities in a great variety of 

 plants, and give the special value to a number of species or 

 genera useful to man. These juices are interesting to the phy- 

 siologist, moreover, like the volatile oils, on account of their 

 important connexion with those chemical processes in which 

 oxygen is liberated, since they are either hydro-carbons or poor 

 in oxygen, and their abundance in a given plant stands in rela- 

 tion to the amount of exposure to solar influence. This relation 

 is not confined to the oily or resinous matters, but holds good 

 of the active principles (alkaloids, &c.) mixed with them. The 

 class of Conifers (pines, firs, &c.) appears to form an exception to 

 this relation of resinous secretions to the sun's action, since they 

 belong especially to high latitudes and mountains ; possibly their 



