SECT. XIT. VEGETABLE SAP. 41 5 



the very lowest, contains a variety of metals in infini- 

 tesimal quantities, as lithium, rhodium, and others ; but 

 they are not essential to the welfare of the plants. Iron 

 is the most frequent constituent in very small quanti- 

 ties ; there are also occasional deposits of soda, lime, and 

 a little manganese. All the various substances which 

 enter the vegetable system, are combined in definite pro- 

 portions into an infinite variety of organic compounds in 

 different plants, and in different parts of the same plant, 

 for the decomposed matter is carried by the ascending 

 sap to every part even of the highest trees. Throughout 

 the whole process the law of the division of labour pre- 

 vails ; to each part of a plant, and to each group of 

 cells, its own duty is allotted. 



The vegetable sap, consisting of water, carbonic 

 acid, ammonia, and other substances, which enter the 

 spongy extremities of the roots in a liquid state, rises 

 in the form of a crude fluid through the whole loose 

 texture of herbaceous plants, through both the wood 

 and pith of trees under two years' growth, and in 

 older trees and shrubs it rises through the sap-wood of 

 the stem into the branches, and thence into the leaves, 

 the limit of ascent in all plants, so that in spring, all 

 the cells are full of sap. The vascular ducts are capil- 

 lary tubes, and the cellular tissue is an assemblage of 

 closed cells or sacs, whose wall or cell-membrane is per- 

 meable by liquids ; hence the imbibition of the roots 

 and the rise of sap in the plant are essentially due to 

 capillary attraction acting contrary to gravitation. The 

 ascension of the liquid is inversely as the diameter of the 

 capillary tubes and cells in the stem and branches ; the 

 quantity raised is the same at all heights, and the 

 velocity of ascent is inversely as the height. 6 As soon as 

 the leaves are expanded, they evaporate a quantity of 

 water through their stomata during the day, so that in 



6 Professor Matteucci. 



