EXPERIMENTS ON THE MOTION OF THE SAP OF PLANTS. 37 



on the motion of the sap in plants in consequence of their evaporation in branches 

 covered with foliage, in cut plants as well as in those still provided with roots. 



He shows by the following experiment the influence of the mechanical pressure 

 of a column of water, with and without the help of evaporation. 



To a branch of an apple tree bearing its twigs and leaves, HALES fastened, air- 

 tight, a tube seven feet long. He kept the branch with its twigs and leaves 

 immersed in a large vessel of water, and filled the tube with water. By the pres- 

 sure of the column of water, water was forced into the branch, and in two days 

 the water in the tube had sunk 14| inches. 



On the third day, he took the branch out of the water, and exposed it to free 

 evaporation in the air. The water in the tube fell, in twelve hours, 27 inches. 



To compare the force with which water is driven through the vessels of the 

 wood, by pressure alone, with that produced by pressure and evaporation, he joined 

 an apple branch, 6 feet long, with leaves, and exposed to the air, with a tube 9 

 feet long, which was filled with water. 



- From the pressure caused by the column of water, and by the evaporation going 

 on at the surface of the twigs and leaves, the water fell (Xlth experiment,) in one 

 hour, 36 inches. He now cut off the branch 13 inches below the tube, and placed 

 the portion cut off (with the twigs and leaves) vertically in a vessel of water. 

 This last absorbed, in 30 hours, 18 ounces of water, while the portion of wood 

 remaining in connection with the tube, which was 13 inches long, only allowed 6 

 ounces of water to pass, and that under the pressure of a column of 7 feet of 

 water. 



HALES shows in three other experiments, that the capillary vessels of a plant, 

 alone, and in connection with the uninjured roots, are easily filled with water by 

 capillary attraction, without, however, possessing the power of causing the sap to 

 flow out and to rise in a tube attached.* The motion of the sap, concludes HALES, 

 belongs to the evaporating surface alone ; he proves that it goes on in an unequal 

 degree from the stem, the twigs, the flowers, and fruit, and that the effect of the 

 evaporation stands in a fixed ratio to the temperature and hygrometic state of the 

 air. If the air were moist, but little were absorbed ; the absorption was hardly 

 perceptible on rainy days. 



He opens the second chapter of his Statistics with the following introduction : 



" Having in the first chapter seen many proofs of the great quantities of liquor 

 imbibed and perspired by vegetables, I propose in this, to inquire by what force 

 they do imbibe moisture. Though vegetables (which are inanimate) have not an 

 engine, which by its alternate dilations and contractions, does in animals, forcibly 

 drive the blood through the arteri'es and veins; yet has nature wonderfully contrived 

 other means, most powerfully to raise and keep in motion the sap. 



In his experiment XXL, he exposed one of the chief roots of a pear tree in full 

 growth at a depth of 2 feet, cut off the point of it, and connected the part of the 

 root left in connection with the stem with a tube which he filled with water and 

 closed with mercury. 



In consequence of the evaporation from the surface of the tree, the root absorbed 

 the water in the tube with such a force, that in six minutes the mercury rose to 8 

 inches in the tube. This corresponds to a column of water 9 feet high. 



This force is nearly equal to that with which the blood moves in the great 

 femoral artery of the horse. HALES, in his experiment XXXIV., found the force of 

 the blood in various animals; "By tying those several animals down alive upon their 

 backs, and then laying open the great left crural artery, where it first enters the 

 thigh, I fixed to it (by means of two brass pipes which run one into the other) a 

 glass tube of above 10 feet long, and gth of an inch in diameter in bore. In which 

 tube the blood of one horse rose 8 feet 3 inches, and the blood of another horse 8 

 feet 9 inches. The blood of a little dog 65 high." 



HALES showed by special experiments, that the absorbent force which he pointed 

 out in the root is found also in the stem, in each separate twig, each leaf, and every 

 part of the surface ; and that the motion of the sap continues from the root towards 



* The motion of the sap is caused by the evaporating surfece. 



