CH. v.] THE SAP. 153 



The moistui'e from the soil absorbed by organs 

 ha\'ing such powers of intro-susception and dis- 

 crimination passes up vessels situated in the wood, 

 but especially in the alburnum, impelled by their 

 contractile power, a power so great that it drives 

 the sap from the extremity of a cut vine branch 

 with a force capable of sustaining a column of mer- 

 cury thirty-two inches and a half high. If a proof 

 of their contractile power, evidently resembling the 

 peristaltic motion of the animal power, be required, 

 Dr. Thomson justly refers for such proof to the 

 evidence afforded by milky-juiced plants like the 

 Euphorbia peplis. If the stem of this plant be 

 divided in two places, the juice flows out at both ends 

 so completely, that if it be again bisected between the 

 two former cuts, no more juice will appear. Now it 

 is impossible that these phenomena could take place 

 ^rithout a contraction of the vessels ; for the vessels 

 in that part of the stem which has been detached 

 could not be more than full; and their diameter is 

 so small, that if that diameter continued imaltered, 

 the capillary attraction would be more than sufficient 

 to retain their contents, and, consequently, not a 

 drop would flow out. Since, then, the whole liquid 

 escapes, it must be driven out forcibly, and, con- 

 sequently, the vessels must contract*. 



Thus propelled, the sap is distributed along each 

 ' Thomson's Organic Chemistrj', 988. 



