PLANT PHYSIOLOGY 129 
Deduction.—The root has absorbed the soluble eosin, 
not the insoluble carmine, hence roots can only absorb 
soluble substances. 
94. Summary.—We see then that the root absorbs 
water from the soil, which contains all the necessary 
salts dissolved in it; that only soluble substances can be 
so absorbed, that the physical process by which absorp- 
tion takes place is osmosis; that the food so absorbed is 
passed on to other parts of the plant, partly by osmosis 
and partly by capillarity; and that this process of ab- 
sorption gives rise to the phenomenon of root-pressure. 
95. Insoluble Substances. —Certain salts in the soil, e.g. 
calcium carbonate, are insoluble in water. These are 
acted upon by the acid cell-sap diffused from the root 
during the process of osmosis and thus rendered soluble. 
96. Experiment 6.—To show that roots give out acid. 
Apparatus.—Blue litmus papers, funnel, test tubes, 
blotting-paper. 
Method.—Line some test tubes and also a funnel with 
blotting-paper which must be kept damp. Between the 
paper and the glass place some strips of litmus paper. 
Put some seeds against the litmus paper so that when 
they germinate the roots will touch the paper. Mealie 
seeds germinate quickly and give good results, so do 
Sunflowers. 
Result.—When the roots are 2 or 3 inches long, the 
litmus papers will be seen to have red marks where the 
roots have touched them. 
Deduction.—Since acids turn blue litmus red, the 
roots must have given out acid. Another experiment 
which shows the same thing is to grow some seeds in a 
