DIV. ii PHYSIOLOGY 237 



root, no water is at first forced from the stock ; but, on the other hand, if water is 

 supplied to it the cut surface absorbs it greedily (negative pressure). Only after 

 it is fully saturated does the forcing-out of water commence. In nature root-pressure 

 thus only comes into play when transpiration is greatly lessened, for instance at 

 night when the air is damp and cool. The most, favourable conditions for this 

 phenomenon occur in spring when, on the one hand, the wood is richest in water, 

 and, on the other, the transpiring foliage is not fully developed. On wounding 

 the xylem the sap then oozes in drops out of the vessels and tracheides. A positive 

 root-pressure in trees with foliage appears only to occur in tropical forests. 



That transpiration induces a suction from cell to cell has been pointed out 

 above, and it is clear that this suction will be continued from the parenchymatous 

 cells into the vessels. This suction force can be readily demonstrated. 



A cut shoot placed with its lower end in water shows by remaining fresh that 

 it is able to raise the water to its uppermost twigs. This does not fully exhibit the 

 amount of suction force which the shoot can exert, for if the latter is connected with 

 a long tube filled with water it can support a water column of 2 metres or more in 

 height. If the end of the tube is dipped into mercury even this heavy fluid will 

 be lifted to a considerable height. Strong and otherwise uninjured branches of 

 Conifers are able to raise the mercury to the height of the barometric column, and 

 even higher, without showing signs of wilting. The connection between the end 

 of the shoot and the glass tube must of course be air-tight. Necessary conditions 

 for such a suction are on the one hand an air-tight closing of the water-conducting 

 tracts such as is actually found in the plant, and on the other hand a considerable 

 cohesive power of the fluid to be raised, which is also found to "exist in practice. 

 The conception is thus reached of a pull exerted by transpiration being conducted, 

 owing to the cohesion of the water, to the tips of the roots of a plant. Very 

 considerable traction forces have been demonstrated in the conducting tracts of 

 transpiring plants as is assumed by the COHESION THEORY ( 19 ). This theory is, 

 however, not yet proved. To transmit the suction downwards, the vessels would 

 require to be continuously filled with water, while, in practice, columns formed 

 alternately of air and water are found. When a pull took place the air bubbles 

 would expand, and in practice air under diminished pressure is found in the 

 vessels of actively-transpiring branches. When such vessels are cut across under 

 mercury, this is forced for a considerable distance into the cut vessels by the 

 force of atmospheric pressure. The supporters of the cohesion theory therefore 

 assume that other tracts completely filled with water are present, and that those 

 containing air merely serve as a magazine of water. It is not out of the question 

 that the living elements always present in the neighbourhood of the vessels and 

 tracheides may play a part in the raising of the water. 



(b) The Nutrient Salts 



The nutrient salts which are absorbed by a plant are almost all 

 met with in the ash ; only the compounds of nitrogen are wanting 

 Thus the following table of the nature of the ash of a number of 

 cultivated plants affords some insight into the amount and the dis- 

 tribution of the nutrient salts. 



It is seen from this table that the ash - constituents are very 

 generally distributed but occur in varying proportions in different 



