CIRCULATION OF THE SAP. 145 



takes place, first from the lower parts of the incisions, and chiefly 

 from the alburnum ; while at a later period of the year it occurs on 

 both sides of the incision, chiefly from the new wood on the lower 

 side, and from the bark on the upper side. If a plant be plunged 

 into a weak solution of acetate of lead (which is capable of being 

 absorbed), the metal may be detected by means of a salt of iodine, 

 first in the new wood, next in the leaves, and then in the bark. A 

 similar experiment may be made by means of weak solutions of potassic 

 ferrocyanide, and of a persalt of iron. 



From the minuteness of the tissue, and the difficulty of examining 

 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 intercellular spaces, 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, the descent of the sap takes place by the cells 

 of the medullary rays. 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. 



Gaseous matters are taken up by the roots of plants, and circulated 

 along with the sap as' well as in the spiral vessels. These usually 

 consist of air, carbonic acid, and oxygen. Hales showed the existence 

 of air in the vessels of the Vine, and Geiger and Proust proved that 

 the sap of this plant contained carbonic acid. Some plants, as Ponte- 

 deria and Trapa, float in water by means of air contained in the vessels 

 or in the intercellular spaces. 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. 



Changes take place in the composition an^ density of the sap in 

 its upward course. The chief alterations in it take place in the 

 Jeaves, where it is exposed to the influence of light and air. Sy this 

 means carbon is fixed, oxygen is given off, and an exhalation of 

 watery fluids takes place. The fluids pass from cell to cell through 

 the leaves, where they are acted upon by air through the stomata, 

 and reach the vascular and cellular tissue of the bark, where further 

 changes take place. Walker, from his experiments, concluded that no 

 descent takes place until after the development of the leaves. 



The sap, after being elaborated in the leaves, is sometimes clear 

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

 opaque. The elaborated sap has been called latex, and the vessels 

 transmitting it have been denominated laticiferous (p. 21). The 

 latex contains granules, which exhibit certain movements under the 

 microscope. The movements are analogous to those observed in the 



L 



