LIFE OF THE WHOLE PLANT. 801 



the amount of light as we can 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 important 

 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 different 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 Umbelli- 

 ferous plants, as Cicuta virosa (Water Hemlock), and (Enanthe 

 crocata (Hemlock Water Dropwort), which are poisonous in 

 most districts of England, are innocuous when grown near 

 Edinburgh. The causes of such differences are at present 

 obscure, but the varying conditions of soil and moisture under 

 which 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 cannot be doubted, but 

 that each plant will only form its proper secretions when grown 

 under those circumstances which are natural to it, and that 

 consequently any change from those conditions will modify to 

 some extent the properties of the plant. Probably 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. 



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

 into the elaborated sap in the manner already described, it 

 passes from the leaves to the stem, probably to the inner-bark 

 and cambium-layer of Dicotyledons ; and apparently to the 

 parenchymatous tissues generally of the stems of Monocotyledons 

 and Acrogens. It then descends in the stems of the several 

 kinds of plants as far as the root, and in its course affords mate- 

 rials for the development of new tissues and the production of 

 flowers and fruit, and at the same time deposits its various 

 secretions. Hoffman in his experiments upon Eerns, however, 

 could not find any path by which the elaborated juices descended 

 in the stem. 



In Dicotyledons, the elaborated sap is commonly believed to 

 3f 



