224 CHEMISTRY OF PLANT LIFE 



reaction, etc., and may be controlled by various stimuli of chemical, 

 physical, or mechanical nature. This conception seems, 'there- 

 fore, to fit most closely the actual conditions under which the 

 protoplasm exists and carries on its vital functions. 



With this conception in mind, we may now proceed to a con- 

 sideration of how the various components of the complex organic 

 colloidal system, and their specific properties, can affect its chem- 

 ical activities. 



The components of the system are, of course, water, salts, and 

 the various organic compounds (fats, proteins, carbohydrates, and 

 enzymes in all cells; and other groups, such as essential oils, 

 tannins, pigments, etc., in cells which have certain special func- 

 tions to perform) which constitute the solid phase of the colloidal 

 mixture. In addition to the definite chemical properties of each 

 of these component groups, 'which have been studied in detail in 

 preceding chapters, there are many physical, or physical-chemical, 

 properties of the system as a whole, and of its component parts, 

 which are of the utmost importance in the physiological activities 

 of the protoplasm. These we may now proceed to consider in some 

 detail. 



WATER 



Water constitutes the largest proportion of the weight of active 

 protoplasm. In living cell contents (except those of such bodies 

 as resting seeds, etc.), water comprises from 70 to 95 per cent of 

 the total weight of the substance; the average proportion being 

 usually between 85 and 90 per cent. The fact that protoplasmic 

 material can exist in turgid form with such high percentages of 

 water as these is due, as has been pointed out, to its existence as a 

 colloidal gel. It is because of this condition that increases in the 

 proportion of water generally increase the turgidity, or turgor, of 

 the protoplasm; instead of, as in all other cases, rendering the 

 mixture less solid and more labile. Losses of water from the 

 protoplasmic gel decrease its " swollen " condition and so render 

 the tissue soft and flabby; while increases in water content swell 

 the gel and make the tissue stiff and turgid. No other condition 

 than that of a colloidal gel could respond in this way to changes in 

 water content. 



The formula which is commonly assigned to water is the sim- 



