238 PLANT PHYSIOLOGY 



no special value to it if they (or their ions) are in higher concentra- 

 tion in the surrounding soil solution and if the cell membranes are 

 permeable to them. Thus peas, oats, radishes, and buckwheat all 

 increase in iodine content when potassium iodide is added to the 

 culture solution even though these ions may be of no value to the 

 plant (Kelly and Stuart, 1928). We have also seen (Chap. X) that 

 plants contain many elements that are of no known value and 

 which they can get along perfectly well without. 



But the entrance of salts and ions is not a question of physical 

 diffusion only. In some plants materials seem to be taken in when 

 there is already a larger concentration inside the cell than in the 

 liquid outside. Hoagland and others found in the alga, Nitella, 

 that the concentrations of Na, Ca, Mg, CI, S0 4 , and P0 4 were 

 all higher inside the cell sap than in the surrounding water. A 

 possible explanation for this is that the ions (or molecules?) are 

 absorbed and then combined in an " inactive" state so that more 

 can then enter. On the other hand, it is possible that under certain 

 conditions (very imperfectly understood) the protoplast may ac- 

 tively absorb certain ions against a concentration gradient. This 

 means that the laws of diffusion do not operate as with a dead 

 substance but that the living protoplasm plays an active role in 

 determining what materials shall or shall not enter. 



These conclusions agree with the known facts that two plant 

 species growing in the same soil will absorb different elements in 

 different amounts according to the " natural needs" of the par- 

 ticular species. This specific character of the protoplasm may be 

 inherited the same as any other character as in the case of Egyptian 

 cotton, which absorbs chlorides more readily than Upland cotton; 

 while the latter absorbs sulphates more easily than does the 

 Egyptian. When hybridized, this character blends in the first 

 generation, but in the second generation shows a Mendelian segre- 

 gation (Harris, 1924-1925). 



Measurement of Osmotic Pressure. — The osmotic pressure is 

 commonly measured in one of three ways. First, one can measure 

 the hydrostatic pressure developed. This, including the plasmo- 

 lytic method, is an indirect one and has already been described. 

 Simpler methods which depend upon physical changes connected 

 with the number of osmotically active particles in solution have 

 been developed. One of these is the determination of the boiling 

 point. The higher the osmotic pressure, i. e., the greater the num- 



