232 A TEXT-BOOK OF BOTANY. 



With the exception of the seeds, in which they are seldom 

 found, volatile oils occur in nearly all parts of the plant. They 

 are formed either as a direct result of the activities of the pro- 

 toplasm or by reason of changes in some of the constituents of 

 the cell-wall. In a few instances the volatile oil is formed from 

 a mother substance, being in the nature of a glucoside, and in this 

 form occurs in the seeds of the almond and mustard. 



Botanical Classification. — The composition of volatile 

 oils is in many cases very complex ; seldom do they consist of 

 only one substance, as in turpentine oil. Usually they consist of 

 a number of chemical compounds, the most complex being 

 American peppermint oil, from which no less than seventeen 

 different, well-characterized chemical compounds have been 

 isolated. As the volatile oils are of considerable economic value, 

 they have been rather extensively studied. It remains for 

 botanists to apply this knowledge to the study of the living plant. 

 The physiologist will find the study of the origin, transportation, 

 and localization of volatile oils in different parts of the same 

 plant of very great interest. Such studies will throw considerable 

 light upon the entire question of origin and transformation of the 

 different plant constituents. In many cases, even the constitution 

 of the constituents in volatile oils has been ascertained, so that 

 on a sound scientific basis, hypotheses may be developed con- 

 cerning the complex changes which are possible in the substances 

 derived from the protoplasm. Again, the distillation products 

 obtained in the stu-dy of volatile oil show that the living plant 

 may contain such simple compounds as formic alcohol, formalde- 

 hyde, formic acid, hydrocyanic acid, etc. 



Volatile oils which have been carefully studied are obtained 

 from plants of the following families : Polypodiacese, Pinacese, 

 Pandanacese, Graminese, Palmse, Araceae, Liliacese, Iridacese, Zingi- 

 beracese, Piperaceae, Salicaceae, Myricaceae, Juglandacese, Betu- 

 laceae, Moraceae, Aristolochiaceae, Chenopodiaceae, Ranunculaceae, 

 Magnoliacese, Anonaceae, Myristicaceae, Monimiaceae, Lauraceae, 

 Cruciferae, Resedaceae, Hamamelidaceae, Rosaceae, Leguminosae, 

 Geraniaceae, Tropaeolaceae, Erythroxylaceae, Zygophyllaceae. 

 Rutaceae, Burseraceae, Meliaceae, Polygalaceae, Euphorbiaceae, 

 Anacardiaceae, Vitaceae, Tiliaqeae, Malvaceae, Theaceae, Diptero- 



