SECT. II 



PHYSIOLOGY 



205 



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 it 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 dip^jed 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 tlie 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. For this, 

 however, the vessels would require to be continuously filled with water, while, in 

 practice, columns formed alternately of air and water are found. If this held for 

 all the water- conducting tracts it would hardly be possible to consider the 

 "cohesion theory " (^'') further. 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. 



Recent work has supported the conclusion that the living elements always 

 present in the neighbourhood of the vessels and tracheides may play a part in the 

 raising of the water. It is not possible to form a clear conception, however, of the 

 part the living cells play in the process. 



(b) The Nutrient Salts 



The mineral substances which form the ash, were at first regarded 

 as accidental impurities of the organic substance of the plant. But 

 every attempt to oljtain a plant free from mineral substances shows 

 that they form essential constituents of the substance. 



It was first asserted by Berthollet (1803), and afterwards 

 emphasised by Karl Sprengel, that the mineral salts contained in 

 plants were essential constituents of plant food. Conclusive proof of 

 this important fact was first obtained in 1842 by the investigations of 

 Wiegmann and Polstorff (^^). 



This conclusion can be reached by two methods, which at the 

 same time show whether all or only certain of the substances in 

 the ash are necessary. The first method is to cultivate the plant 

 in an artificial soil composed of insoluble substances such as platinum, 

 pure carbon, pure quartz, with which the substances to be investi- 

 gated can be mixed. The second method, that of water-culture (^^), 

 is more convenient. Many plants are able to develop their root-system 

 in water instead of in the earth. It is thus possible to add to the 

 water the elements found in the ash in various combinations, and so 



