oS MOLECULAR FORCES IN THE PLANT. 



it has been cut off in air, although that which is wanted for the transpiration of a few 

 leaves can be. 



The cause of this phenomenon is therefore a diminution in the power of conducting 

 water in a short piece above the cut surface of the stem. This is evidently occasioned 

 by the loss of water from the cells caused by the suction of the higher parts not being 

 compensated by absorption from below. All circumstances which favour this loss of 

 water increase also the loss of power of conducting it, and cause the shoot w^hich is 

 placed in water to wither more rapidly and completely. It must therefore be assumed 

 that the conducting power of the cells depends on the quantity of water they contain. 

 The probability of this hypothesis is increased by the fact that by artificially increasing 

 the amount of water in the cells of this piece, its conducting capacity can also be in- 

 creased, as is proved by forcing in water from below. If the modified portion is dipped 

 in water of from 35° to 40° C, the withered shoots soon revive, and if then placed in 

 water of 20 G°. remain fresh for days (as in the case of the elder), or at least wither 

 more slowly [e. g. the artichoke). 



{d) Water retained in the ^wood hy Capillary Attraction. If the capillarity of the cavities 

 in the wood must be considered as w^ithout any immediate action on the currents of 

 water, this force must nevertheless be taken into account with respect to other processes 

 connected indirectly with the movement of water in the plant. In winter and after 

 long-continued rain in summer a large quantity of water is found in the cavities of the 

 wood together with bubbles of air which occupy the wider spaces. It is not known how 

 this water has reached the higher parts of the trees, though it is possibly by the forma- 

 tion of dew as the temperature varies ; it is however to a great extent retained by 

 capillarity. A part of the w-ater flows out in many cases through holes bored in the 

 stem if they are not placed too high, as in the birch, maple, vine, &c. It may be sup- 

 posed that the water which flows out has been forced up by the root-pressure which 

 must also be taken into account ; though how far up this pressure extends is not yet 

 ascertained. The water which does not flow out of the cavities when there is less 

 transpiration 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 w^ater escapes when pieces of wood which have been cut off 

 in cold weather are warmed, because the air expands and forces out the water. Sub- 

 sequent 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. 



(^) The ascent of water from the root into the 3tem^. 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, Ricinus, 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^. 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. 



