230 A TEXTBOOK OF BOTANY \Ch. V, 4 



pressures are much lower than this, usually not more than 

 10 to 20 atmospheres, though in the lower plants, especially 

 some Molds and Bacteria, there is reason to beheve that the 

 pressures rise often far above the 24 atmospheres just men- 

 tioned. 



Such striking and important phenomena as osmotic ab- 

 sorption and pressure demand explanation, which, however, 

 cannot yet be given wdth certainty. A close quantitative 

 relation exists between osmotic pressure and gas pressure, 

 on which account some investigators have considered them 

 identical, holding that a substance in solution is virtually 

 in the state of a compressed gas, and exerts a gaseous pres- 

 sure. Others, however, maintain that nothing more is involved 

 than the adhesive affinity of the sugar, or other dissolved 

 substance, for the water, — the substance confined within 

 the membrane ch'awing and holding the water which 

 can pass the memljrane freely. The most probable explana- 

 tion makes it a result of the checked diffusive power of the 

 dissolved substance, which cannot escape through the mem- 

 brane, though the water can enter. As to the passage of 

 water, and (in case of some membranes) dissolved substances, 

 through membranes which seem perfectly sohd, that clearly 

 occurs between the ultimate structural units of the mem- 

 brane, whether molecules or other units. But the subject is 

 too recondite for further discussion at this place. 



The mineral matters needed by plants are compounds 

 which contain the followng seven elements, — viz. ni- 

 trogen (which plants cannot absorb from its uncombined 

 state in the air, and therefore must obtain from compounds 

 in the soil) ; sulphur and phosphorus, integral constituents 

 of proteins, and therefore of living protoplasm ; potassium, 

 needed for incidental processes in connection with the forma- 

 tion of carbohj^drates ; calcium., a neutralizer of injurious 

 substances; magnesium, an integral constituent of chloro- 

 phyll, with iron, incidentally necessary in some way to the 

 formation thereof. These elements all occur in mineral 



