CHAPTER XXV 



ABSORPTION OF FOOD BY PLANT AND ASCENT OF SAP 



Parallelism between responsive reactions of root and digestive organ Alternating 

 phases of secretion and absorption Association of absorptive process with 

 ascent of sap Electrical response of young and old roots Different phasic 

 reactions, as in pitcher of Nepenthe Response to chemical stimulation 

 Different theories of ascent of sap Physical versus excitatory theories 

 Objections to excitatory theory Assumption that wood dead unjustified 

 Demonstration of excitatory electrical response of sap-wood Strasburger's 

 experiments on effect of poisons on ascent of sap Current inference unjus- 

 tified. 



WE have seen in the last chapter that in the digestive pro- 

 cess as a whole there must be alternating phases of secretion 

 and absorption. The secretion of dissolving fluids, by which 

 insoluble substances are rendered soluble, we found to take 

 place under stimulation, and to be succeeded by a process 

 of absorption, by means of which the now dissolved food- 

 material found access into the organism. These functions, 

 though seen characteristically in the digestive organs of 

 animals, are also to be observed in some plants, such as the 

 pitcher of Nepenthe^ or the leaf of Drosera. Here, situated 

 externally, we find what are practically open stomachs, 

 digesting, as do those of animals, solid organic food. But 

 plants in general have to depend on the supply of inorganic 

 food-material, often presented in solid or insoluble forms, for 

 their nourishment. In this case also it is obvious that the 

 same sequence of solution by dissolving fluids, and subse- 

 quent absorption, must be gone through. And the organ by 

 which this takes place must evidently be the root. In this 

 regard the well-known experiments on the corrosion of 

 marble by the root of a growing plant are sufficient to show 



