SECRETION. 581 



crystals of carbonate of lims : they are especially common in Urticacese, 

 as in Ficus elastica, Morus, Itroussonetia, &c. 



Sugar, commonly occurring as one of the soluble forms of the assimi- 

 lated ternary substances, is occasionally excreted, especially from the parts 

 of flowers, such as the so-called nectaries. Through evaporation of water 

 the sugar sometimes appears in a crystalline form. Grape-sugar (glucose) 

 is apparently formed in the leaves by the combination of carbonic oxide 

 and hydrogen, the former derived from the breaking up of carbonic dioxide, 

 the latter derived from water. At other times it is formed from the me- 

 tamorphosis of starch. It is supposed that from the glucose formed in 

 the leaves the principal carbohydrates, such as cellulose, cuticular sub- 

 stance, some acids (as oxalic and formic), are derived from the oxidation 

 of the glucose. 



Sugar occurs commonly in the corolla-tubes of monopetalous flowers 

 (Lilac c.), on the nectariferous coronet of various plants, on the glands 

 of petals like those of Ranuncuhis, Parnassia, &c., or in pits in the same 

 situation, as in some Liliacese. On the leaves of various species of Acacia 

 occur glands secreting sugar ; and the same is the case in species of 

 Clerodendron, Laurustinus, the lower surface of young leaves of Primus 

 Laurocerasus, &c. Various species of Ash (Fraxinus) and Tamarix excrete 

 a great quantity of saccharine substance under the form of manna. 



The wounds inflicted by insects (Aphis) also cause excretion of sugar 

 from leaves, forming " honey-dew." 



Pectose and Pectase. Pectose is a gelatinous hydrocarbon insoluble in 

 water, alcohol, and ether, found in unripe fruits, and in fleshy roots like 

 carrots. By the agency of acids it is converted into pectine, which gives 

 the viscid character to cooked fruits. Pectase is a peculiar ferment found 

 in fruits, which transforms pectose into pectosic acid. 



Volatile Oils are extremely numerous. They are ordinarily secreted in 

 glands, either external or internal, situated on the herbaceous parts of 

 plants. They are rarely pure substances, the essential oils usually con- 

 taining dissolved resinous matters, camphor, or active principles of various 

 kinds. The odours of plants and many of their most important qualities 

 depend upon these secretions, which are generally peculiar to particular 

 genera or Orders of plants, and not unfrequently differ in slight degrees, 

 so as to be characteristic of particular species in an Order. The chemistry 

 of the formation of these bodies is still very obscure. Some are hydro- 

 carbons ; others contain oxygen in addition ; and sulphur plays an impor- 

 tant part in many, especially in the Cruciferae. The only general state- 

 ment which can be made is, that the majority of the essential oils contain 

 less oxygen in proportion to carbon and hydrogen than the dextrine and 

 the other neutral ternary compounds, and that their production stands in 

 a certain relation to the access of sunlight to the plants. 



The Labiates with their external epidermal glands, the Hypericaceae 

 and Aurantiaceae with their internal glands, the Umbelliferse with the 

 oleiferous vittse in the fruit, the Terebinthaceae, RutaceaB, &c. are striking 

 instances of the occurrence of essential oils in particular Orders. 



Resins, solid or fluid (balsams), are very varied. They occur chiefly 

 in intercellular passages, or in groups of cells especially devoted to the 

 secretion of these products. Very little is known of the processes of their 



