THE PHYSIOLOGY OF PLANT-LIFE 19 



Another class of substances appearing as end- j)i"od nets 

 are the ethereal oils, which ate present in greater or less 

 extent in all parts of flowering plants; the odours of 

 plants are due to the volatile oils -which tliey contain, 

 and which are of great service ill attracting insects to 

 visit the flowers. Oil of cinnamon and oil of bitter 



A 



cam 



Among the reserve ^matters must be reckoned tlie mis 



fats 



All of these, of 



plant origin, are derived from the seeds Hud fruits of 

 various plants, the reason for their occurrence in these 

 organs being obviously connected wath the role wliich 

 they play in supplying the germinating embryo of the 

 plant with food. In the Linseed plant the reserve food 

 material of its seed Is principally oil, this substance 

 taking the place of starch, which is commonly the prin- 

 cipal reserve food iia most plants. I'he same applies to 

 other seeds used in commerce, e.g. those which supply us 

 with oil of mustard, cotton-seed oil, rape oil, &c. 



We must finally deal with an aspect in the physiology 

 of plant -life which it is important for us to consider. 

 As has just been shown, the activities of a plant are very 

 Cotnplicated and very varied. They are also devoid of 

 perfect constancy, and so the amount of the active prin- 

 ciple in any plant, for the sake of which the plant is 

 gathered, may show considerable variation. Many fac- 

 tors may contribute to this variation, Such as soil, climate, 

 or altitude, and in the cultivation of medicinal plants 

 the possibility of such variations raiist be borne iu mind. 

 It is not possible to tell beforehand in which direction 

 the variatioh is going to set. This can only be found by 

 experiment; and the want of thi8 knowledge iS a severe 

 handicap to a country desiroUs of growing certain medi- 

 cinal plants, and entering into commercial rivalry with 



