126 CIRCULATION OF THE SAP. 



be detected, first in the new wood, next in the leaves, and then in the 

 bark. 



258. From the minuteness of the tissue, and the difficulty of ex- 

 amining the circulation in a living plant, it is not easy to determine 

 the vessels through which the sap moves. In its upward course, it 

 appears to pass through the recent woody tissue and the porous vessels, 

 and in its downward course through the laticiferous vessels and cellular 

 tissue of the bark, being also transmitted laterally through the cells of 

 the medullary rays. In some cases, when the bark has been removed, 

 and the tree continues to live, the descent or fall of the sap takes place 

 by the cells of the medullary rays. In the course of this circulation, 

 the sap nourishes the different organs, its carbonic acid and water are 

 partly decomposed, combinations take place with nitrogen, protoplasm 

 or formative matter is produced, and various secretions are formed in 

 the cells and intercellular passages. 



259. Gaseous matters are taken up by the roots of plants and cir- 

 culated along with the sap, as well as in the spiral vessels. These 

 usually consist of common air, carbonic acid, and oxygen. Hales 

 showed the existence of a large quantity of air in the vessels of the 

 Vine, and Geiger and Proust have proved that the sap of this plant 

 contains much carbonic acid. In some aquatic plants, as Pontederia 

 and Trapa, there is a quantity of air contained in the vessels or inter- 

 cellular spaces, with the view of floating them. In Vallisneria, the 

 large cells in the centre of the leaves are surrounded by air cavities, 

 which are seen as dark lines under the microscope. When cut, the air 

 comes out in bubbles, and this escape will continue under water for 

 several days, from the part of the leaf attached to the plant, when ex- 

 posed to the light. An ounce of air has been collected from two 

 leaves of the plant in six days. This air, as well as that contained in 

 sea-weeds, does not enter by stomata, for none exist, but must be taken 

 up by the cells probably in solution. 



260. Changes take place in the composition and density of the sap 

 in its upward course, but the chief alterations take place in the leaves. 

 There it is exposed to the influence of light and air, by means of which, 

 as will afterwards be seen, carbon and hydrogen are fixed, oxygen is 

 given off, and an exhalation of watery fluids takes place. The sap 

 becomes denser, and consequently the process of endosmose is pro- 

 moted, so that the fluids pass from cell to cell along the upper surface 

 of the leaf, and are gradually propelled into the lower cells, where 

 they are acted upon by the air through the stomata, and are ulti- 

 mately sent into the vascular and cellular tissue of the bark, where 

 further changes take place. 



261. Elaborated or Descending Sap. The elaborated sap is some- 

 times clear and transparent, at other times it is milky or variously 

 coloured and opaque. By Schultz it has been called latex, and the 



