30 CHEMISTRY OF PLANTS. 



13. Besides the substances mentioned in the foregoing para- 

 graphs, there are a countless mass, the smaller part of which only 

 are probably at present known, and which appear generally to exert 

 but a very small influence upon the life of the plant. To these 

 belong the substances called by chemists Alkaloids and Vegetable 

 Acids, Resins, Essential Oils, Colouring Matters, &c. Many of 

 these must be regarded as mere secretions, and this would not be 

 the place even to recount them. They may be sought for in 

 manuals of Chemistry. 



A great part of the vegetable acids, almost all the alkaloids, and many 

 of the resins, are found in cavities (receptacula\ or in the so-called 

 latex-vessels, but never in the plant-cells. Others, as the essential oils 

 and resins, are found in solitary cells, which they exclusively fill, and in 

 which it is impossible for any chemical change to take place, so that the 

 cell appears dead. Many amongst them, under peculiar circumstances, 

 fail to be developed, as the poisonous secretion of hemlock, which is not 

 found in the plant of the Asiatic steppes ; whilst others are substituted, 

 the one for the other, without the vegetation of the plant suffering in the 

 smallest degree. Therefore, in the contemplation of the phenomena of 

 vegetable life, I think they may, in a great measure, be disregarded as 

 unimportant substances. Upon these bodies, then, I have little or no- 

 thing to say, and especially as chemistry has scarcely begun to work 

 upon them. 



