PHYSIOLOGY 201 



Vine, 5 litres in the Birch, and 10-15 (50) litres in the Palms). 

 This water, as analysis shows, is not pure, but leaves on evaporation 

 a residue of inorganic and organic substances. 



If a long glass tube be placed on the root-stump and tightly fastened by 

 rubber tubing, the exuded fluid will be forced up the glass tube to a considerable 

 height. How great the force of this pressure is may be shown by attaching to 

 the stump a manometer (Fig. 187). The column of mercury will in some cases 

 be forced to a height of 50 or 60, and under favourable conditions to 100 or more 

 centimetres (140 cm. in the Birch). These pressures would be sufficient to raise a 

 column of water 6, 8, and 18 metres high. The height to which the fluid can be 

 raised is the less surprising when the much greater forces due to turgescence, 

 which are at the disposal of living cells, is remembered C 23 ). 



If, instead of the effects of the pressure, the volume of water exuded each hour 

 be observed, the remarkable fact will be demonstrated that the roots regularly 

 discharge more water at certain hours than at others (PERIODICITY OF ROOT- 

 PUESSURE). The quantity is greater by night than by day. 



When it was shown that the roots were capable of exercising so great a 

 pressure, it was at first believed that the ascent of the sap to the tops of the 

 highest trees was due to root-pressure. This, however, appears impossible in 

 view of the following considerations. The volume of water supplied by root- 

 pressure is not sufficient to replace the quantity given off by evaporation. On the 

 contrary, during moderately vigorous transpiration, such as takes place on a summer 

 day, the root-pressure is of a negative character. Thus, if an actively transpiring 

 plant be cut off near the root, no outflow of water will take place. On the other 

 hand, the stump will energetically draw in water supplied to it ; and not until it 

 has become saturated does the force of the root-pressure make itself apparent. In 

 plants growing under natural conditions, the root- pressure is only effective on 

 damp, cool days, or at nights, when the transpiration is greatly diminished. In 

 spring, when the roots are beginning their activity, the conditions are most favour- 

 able, the wood is full of water, and the transpiring leaves are not yet unfolded. 

 When the wood is injured at this season " sap" is exuded in drops from the vessels 

 and tracheides. 



The so-called BLEEDING from wounds or cut stems is chiefly due 

 to root-pressure, but it is also augmented by the pressure exerted by 

 the living cells of the wood (wood parenchyma, medullary rays). FOR 



THE LIVING CELLS OF ALL OTHER PARTS OF THE PLANT LIKE THOSE 

 OF THE ROOT ARE UNDER CERTAIN CONDITIONS ABLE TO FORCE 



OUT FLUID. In addition to an excess of water in the plant the 

 phenomenon may be determined or increased by the stimulus of 

 wounding or by the healing processes ; this was shown by MOLISCH for 

 the inflorescences of Palms and in borings made in our native trees. 

 The amount of fluid excreted from the callus tissue in the latter case 

 was small, but was forced out even when the pressure amounted to 

 9 atmospheres. In Schizolobium excelsum Figdor found, in Java, a 

 pressure of 8 atmospheres ( 24 ). The outflowing sap often contains, 

 in addition to numerous salts, considerable quantities of organic 

 substances (dissolved albuminous matter, asparagin, acids, and 

 especially carbohydrates). The amount of saccharine matter in the 



