PRESSUEE OF THE SAP. 265 



no pressure for the root-s3stera, another showed a pressure of 

 twenty centimeters of mercury. 



709. Bleeding is not by any means of universal occurrence in 

 ■wounded plants. Horvath found none in tlie 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 ma^- recommence after the wound 

 is reoi^ened. According to Hartig,' 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 ver^' large : — 



Betula pai)yracea, 24 hours, 63 J lbs. (Clark). 



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



712. Hofmeister lias 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 bad a volume of 1,450 cubic centimeters, 

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



713. The pressure at the cut surface of a plant varies widel^- 

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

 details of an experiment b\' Clark- will indicate the variations 

 in pressure noted during a comparative!}- short time. 



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

 daj's after the maximum flow of sap for this species. . . . The 

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

 oscillations, standing nsuallj' in the morning below [its] zero, 

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

 and rising rapidh' witii the sun until the force indicated was 

 sufficient to sustain a column of water manj' 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. 9.3 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. 



'' Report of the Secretary of the Massachusetts Board of Agiiculture foi 

 1873, p. 187. 



