338 PHYSIOLOGY OF NUTRITION. 



134. Ethereal Oils and Resins. 



Ethereal oils in many cases are certainly not to be regarded as 

 excreta bnt as secretions, i.e. as bodies with definite physiological 

 functions (to attract creatures necessary for the transference of 

 pollen, to keep away injurious creatures, etc.). Similarly many 

 resins must be considered as secretions. 1 



Ethereal oils are frequently present in intercellular spaces. If 

 we examine, e.g., transverse sections from the stem of Ruta 

 graveolens, taking care that they are not too thin, we find under 

 the epidermis a hypoderm followed by green parenchyma. In 

 this last occur here and there spaces filled with a yellowish 

 highly refringent fluid (ethereal oil). Intercellular secretory 

 spaces filled with ethereal oil are likewise readily made out in 

 transverse section of the leaf of Citrus. The ethereal oils dissolve 

 in alcohol. 



Ethereal oils, however, do not only occur in intercellular spaces, 

 but also within cells. Secretory reservoirs of this kind, and 

 especially those relegated by de Bary to the category of " short 

 tubes," because the cells in question are nearly isodiametric, are 

 found in Aristolochia Sipho. If we examine transverse sections 

 from stems about 4 mm. thick, we see at once the pith and the 

 vascular bundles with their wood and bast. Each vascular 

 bundle is bounded on the outside by cortical parenchyma, this 

 being followed by a closed ring of sclerenchyma, which projects 

 inwards somewhat between the vascular bundles. Outside this 

 ring of sclerenchyma comes parenchyma, then collenchyma, and 

 finally the epidermis. In the cortical parenchyma lying outside 

 and inside the ring of sclerenchyma, we now observe, both in 

 transverse and longitudinal sections, scattered cells with yellowish 

 highly refringent contents. These are the secretory reservoirs 

 with which we are concerned. Toward alkanna tincture and Osmic 

 acid the ethereal oils behave like fats. If, however, without 

 putting on a cover-glass, we heat the sections on the slide for ten 

 minutes in the drying chamber at a temperature of 130, the 

 ethereal oils disappear, since they are volatile. 



It is further instructive to verify the fact that the fruits of 

 many Umbellifera3 are rich in ethereal oil, occurring here in 

 intercellular spaces, and evidently functioning as a protection 

 against injurious animals. We prepare, e.g., transverse sections 

 through the laterally compressed fruit of Carum Carvi. Each 



