206 BOTANY 



in lignified cell walls, and especially in the cambial sap of the Conifers, is also in- 

 cluded in the glucosides. Coniferin has recently acquired an economic value, as 

 from it vanillin, the aromatic principle of vanilla, may be artificially produced. 

 In this process the coniferin is decomposed, through the action of a ferment or acid, 

 into glucose and coniferylalcohol, through the oxidation of which its aldehyde, 

 vanillin, is formed. 



It is as yet unknown what part in the metabolic processes of plants is per- 

 formed by the bitter principles, such as the lupulin of Hops, aloin of Aloes, 

 absynthin of Wormwood. There is the same uncertainty with regard to the 

 functions of the alkaloids. Since most alkaloids, strychnin, brucin, veratrin, 



OONIIN, MITSCARIN, ATROPIN, QUININ, MORPHIN, CODEIN, COFFEIN (theln), THEO- 



bromin, aconitin, nicotin, PILOOARPIN, oocaine, etc., are violent poisons, their 

 vegetable bases and repugnant bitter principles furnish a certain protection 

 to plants against destructive animals. This, however, does not preclude the 

 possibility that they, like the poisonous oxalic acids, may at the same time 

 have an important physiological significance. 



The colouring matters and ethereal oils, although in actual weight present 

 only in small quantities, make themselves particularly noticeable to the senses 

 of sight and smell. They probably represent only by- and end-products of meta- 

 bolism ; and, with the exception of chlorophyll, take no further part in the vital 

 processes of plants, except in so far as they are beneficial to the general well- 

 being by enticing (e.g. flowers, fruits) or repelling animals. Their biological signifi- 

 cance is accordingly much better known than their physiological function. Just as 

 the ethereal oils are frequently found in special excretory receptacles, the resins, 

 gum-resins, and gum-mucilages, which are also excretion products, are usually 

 deposited in canals or glandular cavities (p. 88), and are often mixed with ethereal 

 oils. Whether their formation in the particular instances is necessary for the 

 carrying out of the normal processes of metabolism is altogether uncertain. They 

 are, at any rate, useful to plants when wounded, serving as a protection against 

 evaporation and the attacks of parasites. 



The significance of the so-called india-rubber (caoutchouc) and gutta-percha 

 (in the latex, p. 73) in the economy of the plant is still less known. In addition to 

 these substances, there also occur in latex, resins, ethereal oils, alkaloids (in opium), 

 starch grains and other carbohydrates, oil- drops and albuminous substances. 

 The presence of these substances in the latex, valuable as constructive material, 

 and occasionally also of active enzymes (peptonising ferments are found in the 

 milky juice of Ficus Carica and Carica Papaya), gave rise to the suggestion that 

 the latex cells and tubes function in the transport of the nutrient matter. It has, 

 however, been found that, even in starved plants, the latex remains unconsumed ; 

 and the present knowledge of these often caustic and poisonous saps is limited to 

 their external utility in the economy of plant life. By their obnoxious properties 

 they defend plants from the attacks of enemies. Also, in the event of plants 

 being wounded, the latex is pressed out either by the surrounding turgescent tissue 

 or by the tension of the elastic walls of its own cells, and forms, as it quickly 

 coagulates in the air, an efficient covering for the wound. In other plants, especi- 

 ally in trees, wound-gum serves the same purpose (p. 81). 



Special Processes of Nutrition 



Parasites, Saprophytes, Symbionts, and Insectivorous Plants. — 



The acquisition of organic nutritive substances through the activity of 



