CHAPTER IV 



IMPORTANCE OF WATER IN THE LIFE OF THE CELL. 

 THE CELL AS A COLLOIDAL OSMOTIC SYSTEM 



33. Saturation of the Cell with Water as an Essential Con- 

 dition of Life. The Water Content in Different Parts of the 

 Plant. — Organic life originated in water, and the most primitive 

 organisms such as amoebae and algae are able to live and to develop 

 only in a water medium. The migration from water to dry land, 

 which finally transformed the algae into higher flowering plants, 

 entailed a series of complex adjustments in the structure and 

 reproduction of plants. The principal physiological properties 

 of living matter, however, have remained the same. The cell 

 of the land plant requires for its normal function almost the same 

 degree of saturation with water as the cell of the submerged plant. 



Of all the substances surrounding the plant, water occupies a 

 prominent position. It is the most important ingredient of the 

 body of a plant, constituting 90 per cent or more of the green weight. 

 Yet, in a strict sense, water is not an organic compound and there- 

 fore not a permanent constituent of the cell. It continually changes, 

 being utilized in the process of evaporation, and then again being 

 replenished from the soil. In other words, water is a part of the 

 environment, permeating the whole organism, and it is in water 

 that all the complex chemical and physical processes take place, 

 the sum total of which is called life. 



The amount of water in the different parts of the plant fluc- 

 tuates within rather wide limits. In organs consisting chiefly of 

 active cells, such as the leaves and the young tips of roots and 

 stems, the water content approaches 90 to 95 per cent. In the 

 wood, where the majority of cells are made up of dead, more or 

 less thickened walls, the amount of water is but 50 per cent, while 

 in such dormant organs as buds and seeds the water content is 

 still lower. Thus, in wintering buds in which feeble vital processes 

 are still going on, 40 to 50 per cent of water may be found. In per- 



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