90 THE GERM THEORY OF DISEASE. 



sally started with the thought that the roots of the plant 

 absorbed water, with whatever might happen to be dissolved 

 in it, and from this the plant appropriated that which it could 

 use in the building of its tissues. This mode of reasoning 

 has, however, failed to give satisfaction. Many facts have 

 appeared from time to time which could not be explained 

 upon this supposition. Prominent among these is the fact 

 that material is continually found in plants which cannot be 

 found in a soluble state in the ground in which the plants 

 grow. Sachs (Text-Book of Botany, p. 625), in discussing this 

 point, says: "But a large portion of food material, especially 

 compounds of ammonia, potassium and phosphoric acid, occur 

 in the ground in a fixed conditioner as it is generally termed, 

 absorbed ; they are not extracted from the soil by very large 

 quantities of water; the roots, nevertheless, take them up with 

 ease. It may be supposed, in these cases, that the absorbed 

 food materials occur as an extremely fine coating over the 

 particles of soil, and can therefore only be taken up together 

 with them by the root hairs at the points of contact; and they 

 are there rendered soluble by the carbon dioxide exhaled by 

 the roots. This action of the roots is limited to the points of 

 contact; only those absorbed particles of substance which come 

 directly into contact with the root hairs are dissolved and 

 sucked up. But, since the numbers and length of the roots is 

 very considerable in all growing land plants, and since, also, 

 they are continually lengthening and forming new root hairs, 

 the root system comes gradually in contact with innumerable 

 particles of earth, and can thus take up the necessary quantity 

 of the substance in question. This power of the roots of tak- 

 ing up, by means of the acid sap, which permeates the walls 

 of even their superficial cells, substances which are insoluble 

 in pure water, presents itself in an extremely evident manner, 

 as I was the first to show. 



