LIFE OF THE WHOLE PLANT. 785 



country, that secretion is not formed at all, or in diminished 

 quantity. Even if such plants be placed in our hot-houses, 

 where they may be submitted to the same degree of heat as they 

 obtain naturally in their native countries, their secretions are not 

 formed, because light is the main agent concerned in their for- 

 mation, and we cannot increase the amount of light like heat by 

 artificial means. 



Another cause which commonly interferes with the formation 

 of the secretions of plants of warmer regions when grown in our 

 hot-houses, is the want of a proper and incessant supply of fresh 

 air to facilitate transpiration, &c. 



The above facts are of great interest, as they have an im- 

 portant bearing upon the growth of plants and fruits for the 

 table, as well as in a medicinal and economical point of view. At 

 present, however, much remains to be discovered, before we can 

 be said to have anything like a satisfactory explanation of the 

 causes which influence the formation of the secretions of plants ; 

 for it is found that the same plants when grown in diiferent 

 parts of Great Britain, where the climatal differences are not 

 strikingly at variance, or even at the distance of a few miles, or 

 in some cases a few yards, frequently vary much as regards the 

 nature of their peculiar secretions. A striking illustration of this 

 fact is mentioned by Dr. Christison, who found that some Um- 

 belliferous plants, as Cicuta virosa (Water Hemlock), and Hem- 

 lock Water Dropwort {(Enanthe crocata), which are poisonous 

 in most districts of England, are innocuous when grown near 

 Edinburgh. The causes which lead to such differences are 

 at present obscure, but the varying conditions of soil and 

 moisture under which such plants are grown, have doubtless an ' 

 important influence upon their secretions. In a Pharmaceutical 

 point of view, as far as the active properties of the various 

 medicinal preparations obtained from plants are concerned, this 

 modification in the secretions of plants by such causes is of 

 much interest, and would amply repay investigation ; for it can- 

 not be doubted, but that each plant wUl only form its proper 

 secretions when grown under those circumstances which are 

 natural to it, and that consequently any change from those con- 

 ditions will modify to some extent the properties of the plant. I 

 cannot but believe that here we have an explanation, to some 

 extent at least, of the cause of the varying strength of medicinal 

 preparations obtained from plants grown in different parts of this 

 country, or in different soils, &c. We cannot, however, further 

 discuss this matter here; the above is only thrown out as a 

 suggestion for ftiture inquiries. 



Descent of the Sap. — After the crude sap has been transformed 



into the elaborated sap in the manner akeady described, it is 



returned from the leaves to the inner-bark and cambium region 



of Dicotyledons ; and apparently to the parenchymatous tissues 



3£ 2 



