4i 6 SAP MOTION. PART n. 



a tree or any plant, an enormous extent of evaporating 

 surface aids in raising the sap by creating a vacuum in 

 all the upper cells and vessels, by which the force of 

 suction and the rapidity of ascent are increased. It 

 appears that the water evaporated by the leaves is in 

 exact proportion to that taken up by the roots to sup- 

 ply its place ; but as soon as the young branches are 

 formed, the buds for the following year produced, and 

 when the leaves are full of the chlorophyll which 

 they have consolidated during the summer, the evapo- 

 ration is less, the sap ceases to rise, the spirals and 

 vascular ducts in the medullary canal and sap-wood 

 are left dry, and fill with air, which they convey to 

 every part of the plant except the bark, to assist in 

 assimilation, that is, in the formation of organic 

 compounds. 



During the whole of this process the leaves and other 

 green parts, which are the organs of vegetable respira- 

 tion, are most active. They absorb carbonic acid gas 

 from the atmosphere by day, and exhale oxygen. For 

 by the direct action of solar light the carbonic acid gas 

 and ammonia in the crude sap are decomposed, part of 

 the oxygen is set free and exhaled, and the rest, with 

 part of the remaining elements, combine to -form chloro- 

 phyll, which is a compound of starch and a little nitro- 

 gen. The oxygen inhaled by plants during the night, 

 combined with other elements in the sap, forms oxidized 

 vegetable compounds. 



M. Kosmann, of Strasburg, observed that both the 

 leaves of plants and their corollas give out a ponderable 

 quantity of ozonized oxygen, much more than that which 

 exists in the air, and that the quantity is less in the 

 night. 



All parts of plants that are not green exhale carbonic 

 acid gas, and inhale oxygen, like animals, night and 

 day ; if prevented from inhaling oxygen they lose their 



