MOVEMENT OF WATER' IN PLANTS. 685 



ascertained. The water which is present in the cavities when there is feeble transpira- 

 tion and which will not flow out of them is clearly retained by capillarity, assisted by 

 the air in the cell-cavities ; for Montgolfier and Jamin have shown that in capillary 

 spaces which contain water and air, the water is not easily set in motion. This explains 

 also the phenomenon already mentioned, that water escapes when pieces of wood which 

 have been cut off in cold weather are warmed, because the air expands and forces out 

 the water. Subsequent cooling causes on the contrary water to be sucked in at the cut 

 surface, because the air contracts, and the pressure of the external air forces in water 

 from without. 



{e) The ascent of <water from the root into the stem *. The most important features of 

 this phenomenon have already been briefly mentioned. It is to be observed in the 

 open air in plants of the most diff"erent kind, if they possess vigorous root-systems and 

 well-developed wood ; as, for instance, in the Birch, Maple, and Vine, and among annual 

 plants, in the Sunflower, Dahlia^ Ricmtis^ Tobacco, Gourd, Maize, Stinging Nettle, &c. In 

 order to study the phenomenon accurately, it is best to grow the plants for some time 

 previously in large flower-pots until they have developed a strong root-system. Land- 

 plants such as Maize grown in water and artificially fed by nutrient substances are 

 also well adapted for the investigation. If the stem of such a plant is cut across smoothly 

 5 or 6 cm. from the ground, and a glass tube fixed to the stump by means of an 

 india-rubber tube, the result will be seen as follows. If the plant was in a condition 

 to transpire freely before it was cut, the cut surface of the root-stump remains at first 

 quite dry, and if water is poured into the glass tube it is at once sucked up 2. The 

 woody substance of the root-stump has evidently been exhausted by transpiration before 

 the operation, and contains but very little water ; not only are its cavities empty, but 

 even the cell-walls of the wood may not be saturated. After a shorter or longer time 

 however the exudation of water at the cut surface begins— rising higher and higher 

 in the tube — and continues from six to ten days if the plant is properly treated, be- 

 coming during the earlier part of the time continually more copious, attaining a maxi- 

 mum, and finally diminishing until it ceases with the death of the root-stock. If the 

 cut section is repeatedly dried with blotting paper during the time that the water is 

 flowing, it is clearly seen that the water exudes from the woody tissue — in Monocoty- 

 ledons from the xylem of the separate bundles — and that it comes principally from the 

 openings of the larger vessels. That the water which flows out had previously been 

 absorbed by the roots out of the ground, and not merely from the store in the root- 

 stock, is at once evident from the fact that the quantity which exudes at the cut section 

 is after a few days greater in volume than the whole of the stock. Under the conditions 

 here described, the water which flows out contains only traces of organic substances 

 in solution ; but the presence of mineral constituents can be easily proved, especially 

 lime, sulphuric acid, phosphoric acid, and chlorine, which the plant has absorbed out of 

 the ground. The water which flows in the spring from holes bored in trees such as the 

 Birch and Maple, contains however considerable quantities of sugar and albuminous sub- 

 stances ; since the longer stagnation in the cavities of the wood gives it the opportunity 

 of absorbing these substances out of the closed living cells of the wood and out of the 

 surrounding parenchyma, a result which cannot be expected, or only in a smaller degree, 

 in the case of the rapid flow from the smaller root-stocks of quickly-growing plants. 



In order to determine the quantity of the outflow, a narrow burette may be used 

 instead of the tube, in which the amount can be read off hourly in cubic centimetres 

 when the outflow is at all considerable. The root-pressure which acts upon the cut 

 surface is however then considerably altered. In order to avoid this, a tube of the 



^ See in particular Hofmeister, On the tension and the quantity and rapidity of the flow of the 

 juices of living plants ; Flora, 1862, p. 97. 



"^ This fact is sufficient to prove that the root-pressure has no share in the ascent of the water at 

 the time when transpiration is active. 



