Water Deficit or Unsatisfied Hydration Capacity. 



95 



Table 71. — Swelling of Rudbeckia. 



of cell-masses, the acids, salts, sugars, etc., in the vacuoles and 

 syneretic cavities of the protoplasm increase in concentration as the 

 dehydration proceeds, until as the end is neared these substances are 

 taken up by the protoplasmic colloids from solutions which may be 

 saturated. 



The sweUing of a living section continues until hydration capacity 

 is satisfied or balanced against the mechanical restraint of the cell- 

 walls, and includes the possibilities of osmotic action by reason of their 

 differential action. The colloids are subjected to the action of the 

 salts and acids only to attenuations in which these substances are 

 usually present. When desiccated sections are swelled, the cell col- 

 loids are now in the condition of having adsorbed substances previously 

 in great dispersion and the external layers of the cells now exercise 

 only the effect of a dense filter- 

 paper; consequently any osmotic 

 pressure which might be set up 

 is soon equalized, and hence con- 

 tributes but little to the ex- 

 pansion of the cells. 



Returning to the matter of the 

 fresh sections of Rudbeckia, the 

 nature of the increase in hydra- 

 tion capacity of the material with the march of development may be 

 set forth more clearly by the figures in table 71 , in which the percent- 

 ages of swelling are multiplied by 10. 



Swelling was greatest in water in both young and old stalks and the 

 increase of the hydrogen-ion concentration or acidity of the medium 

 resulted in reducing the final hydration capacity of the tissues, ir, ore 

 water being taken up when hydroxid was used, but the total 

 capacity being less than in water. The desiccation of the stems, 

 with attendant possible coagulatory effects, was accompanied by an 

 increase in hydration capacity in the acid solutions. The greater 

 swelling shown by old tissues suggests increased pentosan or mucil- 

 age content rather than a deficit due to loss of water. 



No opportunity was lost to make measurements of other material 

 which might possibly show the diversity of action of different plants. 

 The terminal internodes of Verbena ciliata were available at the Desert 

 Laboratory in March 1918, and sections of these with a tangential 

 slice removed were about 1.8 mm. in thickness. Swellings at 18° to 

 20° C. were as shown in table 72. 



Table 72. 



Distilled water 



Citric acid, 0.01 N 



Sodium hydroxid, 0.01 M . 



p. ct^ 

 7 



2.8 

 7 



Potassium chloride, hydrochloric acid, 0.01 M 4 



