— n8 — 



It is very easy to find the layer in dried vegetable material (drugs). 

 In a few cases the cuticle will here be found to have burst, and the 

 secretion to have run out, the layer being thus disclosed without fur- 

 ther trouble; in other cases it is necessary to use chloral hydrate 

 solution, as mentioned above. 



By the aid of the processes described, Tunmann succeeded in 

 discovering the resinogenous layer in all the plants examined by him. 

 In the course of his investigations he was able to determine various 

 typical forms of the layer. These he divides into three principal 

 types: the rod-type [Viola, Fraxinus, Alnus), the vacuola-type (Salvia, 

 Hyssopus) and the mesh or grille type [Rhododendron, Azalea). 



The cuticle of the glands of the skin is partly enlarged by stretch- 

 ing, partly by subsequent development. Its principal purpose is unquest- 

 ionably to prevent a too rapid exudation or loss of the secretions. 

 In the case of all the persistent glands of the Ldbiutce* Pelargoniae, 

 Compositae etc., all of which possess a strong cuticle, a continuous 

 volatilisation of essential oil takes place throughout the whole life of 

 the plant, that is to say, a loss of the volatile portions of the secre- 

 tion by way of the cuticle. In the course of this process the chemical 

 composition of the essential oil must of course undergo some modi- 

 fication, but it does not reach a demonstrable process of resini- 

 fication, because new volatile portions are continuously being formed. 

 Only in autumn, when the period of growth is reaching its end, this 

 formation of volatile constituents ceases, and the remainder of the 

 oil resinifies. Thus it is that autumnal leaves are found to contain 

 in lieu of the usual, almost colourless, highly refractory essential oil, 

 a dark yellow, partly crystalline, partly amorphous, somewhat sparingly 

 soluble lump of resin. 



Generally speaking, the view has been accepted that vegetable 

 secretions are decomposition products formed in the course of 

 the metabolism, but Tschirch breaks a lance for the theory that 

 these secretions are built up to serve quite definite and various 

 biological objects, and in this view he is supported by Tunmann. 



In some cases, the formation of essential oil in the plant begins 

 at a very early stage, in fact, before the gland has attained its full 

 development. 



In opposition to Charabot, Tunmann shows that the constant 

 change in the chemical composition of vegetable essential oils during 

 the progress of the development of the plant, is chiefly due to the 

 continuous evaporation of the more volatile parts. He agrees with 

 Charabot in deducing, from pharmaco - physiological considerations, 

 that plants in flower cannot yield so valuable an oil as can the 

 young spring leaves. 





