PRESSURE OF THE SAP. 265 



no pressure for the root-system, another showed a pressure of 

 twenty centimeters of mercury. 



709. Bleeding is not by an 3- means of universal occurrence in 

 wounded plants. Horvath found none in the following cases: 

 Humulus Lupulus, Hedera Helix, Syringa vulgaris, and Sam- 

 bucus nigra. In some cases there appears to be bleeding only 

 from the cut root, none occurring from the stem. 



710. The bleeding from a plant may be greatest immediately 

 after the wound is made, or it may in a few cases not reach a 

 maximum for some hours or even days, after which it gradually 

 declines until it ceases. It may recommence after the wound 

 is reopened. According to Hartig, 1 bleeding may continue in 

 some cases for a month. 



711. The amount of sap which escapes during bleeding is 

 variable even in the same species. The following cases show 

 that the loss is very large : 



Betula papyracea, 24 hours, 63 Ibs. (Clark). 



Agave Americana, 24 hours, 375 cubic inches (Humboldt). 



712. Hofmeister has given the following example, to show 

 how large is the relative amount of sap which can flow from cer- 

 tain plants. From a specimen of Urtica urens (stinging nettle), 

 whose root-system had a volume of 1,450 cubic centimeters, 

 there escaped in 2J days 11,260 cubic centimeters of sap. 



713. The pressure at the cut surface of a plant varies widely 

 in any given case, according to the surroundings. The following 

 details of an experiment by Clark 2 will indicate the variations 

 in pressure noted during a comparatively short time. 



" A gauge was attached to a sugar-maple March 31st, three 

 days after the maximum flow of sap for this species. . . . The 

 mercury [in the gauge] was subject to constant and singular 

 oscillations, standing usually in the morning below [its] zero, 

 so that there was indicated a powerful suction into the tree, 

 and rising rapidly with the snn until the force indicated was 

 sufficient to sustain a column of water many feet in height. 

 Thus at 6 A. M., April 21st, there was a suction into the tree 

 sufficient to raise a column of water 25.95 feet. As soon as the 

 morning sun shone upon the tree the mercury suddenly began 

 to rise, so that at 8.15 A. M. the pressure outward was enough to 



1 Botanische Zeitung, 1862, p. 89. 



2 JJeport of the Secretary of the Massachusetts Board of Agriculture for 

 1873, p. 187. 



