MOVEMENT OF WATER IN PLANTS. 6oQ 



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

 the operation, and contains but very Httle 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 form 

 shown in Fig. 438 i? (p. 600) is fixed to the stump, and to it is attached a narrow tube 

 instead of the manometer ; the free end of this tube is bent downwards into a graduated 

 burette. If the reservoir is from the first filled with water, as much runs into the 

 burette as flows out from the cut section, and the pressure therefore remains constant. 

 This experiment shows that the flow of water varies from day to day, from one time 

 of the day to another, and even from hour to hour ; but the causes of these variations 

 in the outflow, which must depend on the activity of the roots, are not yet known ; it 

 would even seem as if a periodicity were established independent of the temperature and 

 of the moisture of the ground ^ 



The measurement of the lowest pressure at which the outflow can take place at the 

 cut surface can be effected by the apparatus figured in Fig. 438, where it is expressed 

 by the difference of level of the mercury in the two arms of the tube, or by q—q. 

 This will however only afford a measurement of the pressure which the outflowing water 

 may still have to overcome at the cut surface ; but it has obviously had also to overcome 

 other resistances of unknown magnitude in the interior of the root-stock. With respect 

 to this point I was interested in ascertaining how great is the difference in the outflow if 

 one of two equal root-stocks has no pressure to overcome at the cut surface, the other 

 a considerable but constant pressure. If, in Fig. 440, a indicates the cut stem of a 

 sunflower or similar plant grown in a pot, c, d, e the tube which is attached to it by the 

 india-rubber tube b, and / a glass tube bent downwards, which (not as in the figure) 

 reaches beyond the rim of the pot and terminates in a burette, while the opening of/ 

 lies exactly on the level of the cut surface of the stem; then, when the tube r, d, e,f 

 has been filled with water, we have an apparatus for observing the outflow when the 



^ Very detailed observations on this point have just been made by Baranetzky in the WUrzburg 

 laboratory, in the summer of 1872, 



R r 



