78 The Sap and its Functions. 



and by digging up and burning the infected portions. A dying off 

 of the chestnut trees in Italy, France, and Spain has been attrib- 

 uted to fungous growths upon the roots. A similar cause of decay 

 is noticed in some species of the pine. 



278. The truffle, a fungus that forms over the roots of the oak, 

 in some countries, is a highly esteemed article of food, and becomes 

 at times a product of much profit in the forests where it is produced. 

 This fungus is formed chiefly in calcareous soils, and appears to re- 

 quire certain conditions of climate. In the neighborhood of Avig- 

 non, in France, the revenue from this source alone, in a single com- 

 mune, sometimes exceeds $5,000 a year. 



On the Pressure of Sap, and its Changes. 



279. The moisture of the soil is absorbed by the roots, and pass- 

 ing through these to the trunk, branches, and leaves, descends again, 

 more or less changed, to supply nutriment to every part. The ac- 

 tion of liquids in passing through membranes and cellular tissues, 

 amounting sometimes to a pressure of several atmosphere, is called 

 endosmose, and is shown in a striking manner, when measured by 

 gauges. 



280. About 1720, the Rev. Stephen Hales, an English observer, 

 began a series of experiments upon the pressure of sap, and the ab- 

 sorption of Avater by plants, carefully noting the changes observed, 

 and publishing the results. These experiments form the basis of 

 much that has since been determined in this line of research. 



281. In the spring of 1873, a series of experiments was begun at 

 the Amherst Agricultural College, Mass., by its president (Mr. W. 

 S. Clark) and associates, which led to very interesting results. Sev- 

 eral mercurial gauges were provided, and attached to trees that had 

 been tapped, by screwing in a hollow plug of metal, to which the 

 gauge was connected. The changes in pressure were read upon a 

 scale. It was found that at some hours the pressure was inwards, 

 and at others outwards. In the latter case only would the sap 

 flow from the spouts in the usual way. These gauges, when at- 

 tached to the birch, showed a much greater range of pressure than 

 on the maple, and this even when connected with an isolated root 

 in the ground. 



282. The extreme range in the sugar maple was from -f- 46 to 



