274 TRANSPIRATION. 



off from moist inorganic bodies and exposed liquids. Of the materials which are 

 held in solution in the sap of plants, only those which have the property of passing 

 from the fluid to the gaseous condition, at the same temperature which transforms 

 water into water-vapour, can evaporate with this fluid. All the others remain 

 behind, and the natural consequence is that the sap in the transpiring cells becomes 

 more concentrated. If water, which contains in solution extremely small quantities 

 of sugar, organic acids, nitric, sulphuric and phosphoric acids, and salts of potassium, 

 calcium, and iron, be set to evaporate slowly in a shallow dish, it will gradually 

 come about that only a thin layer of fluid is left on the bottom of the dish; but this 

 now is seen to consist of a very concentrated solution of the substances mentioned; 

 i.e. of the sugar, organic acids, and the various salts. It has also all the properties 

 of such a concentrated solution, i.e. it has the power of sucking in water in the 

 liquid condition from its surroundings. In the same way the contents of a cell in 

 contact with the air become more concentrated by evaporation, and thus obtain the 

 power of abstracting water from the environment of the cell, that is to say, of suck- 

 ing it up. If two adjacent cells contain sap of the same density, whilst only one of 

 them has the power of exhahng water, the condition of equilibrium between them 

 will be destroyed. However, the balance naturally tends to be restored, and the cell 

 whose sap has become more concentrated by the evaporation of water, takes up 

 watery fluid from the neighbouring cell. Now picture a chain of cells containing 

 abundance of sap connected with one another by cell-walls through which fluid can 

 filter, and let them be so arranged that only the uppermost member of the chain is 

 in contact with the atmospheric air. The sap of this uppermost cell having become 

 concentrated by evaporation will first of aU exert a suction on the cell immediately 

 below. As fluid is withdrawn from this second cell, its sap also undergoes concen- 

 tration, and in consequence produces suction on the third cell, the third in like 

 manner on the fourth, the fourth on the fifth, &c., passing from above downwards. 

 In this way innumerable compensating currents are set up between the adjoining 

 cells, which, however, never lead to true equihbrium as long as evaporation con- 

 tinues in the cell in contact with the air, but combine together to form a single 

 ascending stream. 



Such a current actually exists in all living plants which evaporate from the 

 portions above the ground and in contact with the air, while their lower extremities 

 are embedded in a damp nutritious soil. This has been termed the Transpiration 

 Current. Its source is the fluid which has been drawn from the earth by the 

 absorptive cells and brought within the sphere of the living cells of the plant; we 

 may retain for this fluid the old and very appropriate name "crude" or "raw sap". 

 Its direction and destination are determined by the position of the evaporating cells, 

 and its path is through the wood, which in tree-trunks is inserted as a huge layer 

 between the bark and the pith; in lesser stems it passes through the bundles and 

 strands of woody cells and vessels which traverse them, being connected, deep under 

 the ground, by groups of parenchymatous cells, with the absorptive cells of the 

 young rootlets, or with the hyphae of the mycelial mantle, which replace the 



