THE ABSORPTION OF WATER 25 



living state, invariably saturated with water of imbibition ; in the second place, 

 the protoplasm of the living cell is also always saturated with water, and in 

 the third place, the vacuoles, which frequently constitute the greater part of 

 the cell, consist mainly of water. Again, chemical analysis (p. 5) has shown 

 that parts which appear to contain little or no water do, as a matter of fact, 

 contain it in considerable quantities. Were the plant to be as sparing in its 

 use of water as it is of nitrogen (Lect. XI), absorption of water would be pro- 

 portional to the addition of new members to the plant body ; but such economy 

 is by no means the rule. On the contrary, the plant is, at least under certain 

 conditions, lavish in its use of water. The large amount of water which the 

 plant has extracted from the soil by means of the roots, with the expenditure 

 of much energy, it sends back again into the air in the form of water vapour. 

 According to Haberlandt (1877), a plant of maize in the course of the summer 

 gives off into the atmosphere 14 kg. ; a hemp plant gives off 27 kg. ; and 

 a sunflower, 66 kg. ; i. e. in each case several times the weight of the plant. All 

 these plants are small ; what then must be the amount given off by a tree ? 

 The following data, which we owe to v. Hohnel's very careful calculations, 

 may be quoted in this relation. A large birch tree with, say, 200,000 leaves, 

 gives off in the course of the season 7,000 kg., or about 38 kg. per day. A no- 

 year old beech tree gives off, in round numbers, 9,000 kg. in the course of the 

 summer, and, on the basis of 400 such trees to the hectare, a wood of that 

 size (two and a half acres) would give off 3,600,000 kg. of water in one season. 

 Although these numbers make no claim to absolute accuracy, still they 

 give us a rough idea of the very large amount of water concerned in this 

 process. 



Our first task must be to investigate the means by which the plant is able 

 to absorb so vast an amount of water from the soil. With this subject is 

 intimately connected the transpiration of water vapour from the leaves. Since 

 the parts which are concerned in the absorption of water and those con- 

 cerned with its transpiration are situated far apart, to obtain a complete 

 picture of the whole process of water circulation in the land plant we have to 

 consider as well the mode of conduction of water. 



The soil from which the normal terrestrial plant obtains its entire water 

 supply consists of a mixture of mineral detritus and the remains of organisms 

 (humus). The individual constituent particles are of very variable size, loosely 

 or compactly arranged, but in all cases so aggregated that interspaces occur 

 between them, spaces which we will assume, to begin with, are full of air. 

 If rain falls on such a soil, or water reaches it from somewhere else, the 

 air is completely driven out of the interspaces between the soil particles, which 

 then become filled with water. If the subsoil be impermeable to water, for 

 example if it consists of clay, this condition of things becomes permanent 

 and a marsh arises, characterized at once by the abundance of water and the 

 scarcity of air. The abundance of water enables the plant to recoup itself 

 with ease for the water it has lost, and hence it might be imagined that such 

 a soil would form an ideal habitat for a plant. Experience teaches us that 

 this is far from being the case. Certain plants only, and amongst cultivated 

 plants only a few (e. g. rice) are able to thrive in marshy soils, or even tolerate 

 them for any length of time, whilst the majority of our economic plants are 

 killed in the presence of superabundance of water, and thrive only in soils with 

 a moderate supply (Wollny, 1897). The reason for this is, not that there is 

 too much water, but that certain accessory conditions are not fulfilled. That 

 the injurious effects associated with marshy conditions are due to the poisonous 

 influence of putrefying substances is a common belief, but no good evidence in 

 support of this view is as yet forthcoming (Wacker, 1898). The only other 

 possible reason for the injurious influence of a marsh on land plants is the 

 absence of oxygen. As a matter of fact, water-culture experiments teach us 



