272 CAPILLARITY AND ROOT- PRESSURE. 



refreshing beverage. Many Araliaceae also furnish a sap fit for drinking. Some 

 native Indian genera which are used as vegetable wells have on this account 

 received the name of "plant springs" {Phytocrene, e.g. P. gigardea and bracteata). 

 If the young flower-stalk of Agave americana, an American plant which is 

 cultivated in European gardens under the name of the "hundred years' aloe", be 

 cut across, in twenty-four hours about 365 grammes, and in a week more than 

 2500 grammes of sap will flow out. This exudation continues for four to five 

 months, and a vigorous Agave will produce in this time as much as 50 kilogrammes 

 of sap, which will ferment', since it contains both sugar and albuminous substances, 

 and is indeed used by the Americans in the preparation of an intoxicating drink 

 called " pulque". The quantity of sap which exudes from vines is also very great. 

 A branch 2i cm. thick, cut across 1£ m. above the ground, produced within a week 

 over 5 kil. of sap. In a week, from the cut stem of a rose, more than 1 kil. was 

 exuded. From maples and birches a proportionately large amount of sap can be 

 obtained, when the trunks are cut about a metre above the ground. The sap which 

 flows from species of maple contains pure crystallizable sugar, and in some North 

 American species this is present in such abundance that it was found to be worth 

 while to collect the sap, at least in former times. 



It should be noticed that the volume of the exuded sap is in all these cases 

 greater than the volume of the root together with that of the stump of the stem 

 from which the sap is forced out, and this is a proof that it does not consist only of 

 the water which was contained in the root and stem stump at the time of cutting, 

 but that there is a continual upward current of sap, and that the absorbent cells of 

 the roots, for a long time after the operation, continue to draw up water from their 

 environment. 



An ingenious experiment was performed at the beginning of last century in 

 order to ascertain the amount of pressure by means of which the sap is forced from 

 the cut surface of the vine and other stems. A vine stem without branches and 

 about the thickness of one's finger was cut across in the spring at a height of about 

 80 cms. above the ground, and on the root-stock was fixed a glass tube with a 

 double bend, in such a way that one end fitted exactly over the cut surface of the 

 stump, and the tube was then filled with mercury. By the pressure of the sap 

 which welled from the cut surface the mercury was forced up the tube, and in a 

 few days it actually reached a height of 856 mm. The weight of a column of 

 mercury 760 mm. high is equal to that of a column of air as high as the atmosphere 

 of the earth, or of a column of water about 10'3 m. high, and consequently the 

 pressure by which the sap is forced out of the vine is considerably greater than the 

 weight of one atmosphere, or of a column of water of the height mentioned. From 

 these data it has been estimated that the sap can be raised through ll - 6 m. by tin- 

 pressure originating in the absorbent cells of the root. The pressure is naturally 

 greatest in the lower portions of a stem, and gradually diminishes towards the 

 higher regions; the ascending current of sap to which it gives rise is also not 

 uniform, but shows daily, and even hourly, fluctuations. Moreover, the quantity of 



