THEIR DEPENDENCE UPON LIGHT. 227 



themselves. Many plants, which, under the rays of a tropical 

 sun, produce secretions of a powerful character, whether as medi- 

 cines, as aromatics, or as dyes, are almost inert in colder climates, 

 even when the amount of heat artificially given may fully equal 

 that to which they have been accustomed. Thus, the Tobacco 

 of Persia is universally celebrated for its peculiar perfume ; and 

 from the Roses of the South alone is it worth while attempting 

 to obtain the powerful essential oil, which is known as Otto or 

 Atar of Roses. This circumstance is taken advantage of in the 

 growth of vegetables for the table ; for, if they are reared 

 under a diminished light, many kinds of plants may be used as 

 food, which naturally contain secretions, either unpleasant in 

 taste or injurious in character. Such are the Sea-kale, Lettuce, 

 and Cichoiy ; which are prevented from becoming rank, by 

 heaping earth around their young shoots, or by growing the 

 entire plant in a dark situation. The peculiar secretions, too, 

 are not present in young plants, all whose energy seems ex- 

 pended in the extension of their own structure ; hence those 

 kinds, which are afterwards rank poisons, may be eaten with 

 impunity at an early period. Thus the peasants of Languedoc 

 employ young poppies as food ; and. cattle do not reject noxious 

 weeds in spring, which their instinct would not permit them to 

 touch in summer. 



363. As the special secretions of plants are formed in the 

 elaborated sap, they will not be found in those parts, to which 

 it is not afterwards conveyed. They may generally be traced 

 first in the leaves ; but in the course of their descent, they are 

 often separated by some particular organ, in which they are con- 

 centrated (as it were) to the exclusion of the rest. Thus many 

 of the most powerful medicinal agents are obtained from the 

 bark ; some abound most in the roots ; other products, especially 

 resins and colouring substances, seem to be chiefly deposited in 

 the wood ; fixed oils are generally conveyed to the seeds, where 

 they seem to be deposited for the same purpose as starch, the 

 nourishment of the embryo ; whilst aromatic oils are generally 

 found either in the leaves, in the leafy parts of the flower, or in 



