102 PLANT PHYSIOLOGY 



it has been found that the list should be expanded. Whether this 

 list will continue to grow remains to be seen. At the present time, 

 it seems rather that we shall find that there are differences be- 

 tween individual species: certain elements will be found essential 

 for some plants but not for others. 



Since some of these elements are needed in the merest traces, 

 it may be that their absence from the list of essential elements 

 has been due to the use of impure salts in the nutrient solution, 

 where their presence has until recently been undiscovered; or it 

 may develop that their action is largely a balancing one (Chap. XI) 

 and the improvement which they cause in a solution which lacks 

 them is balancing rather than actually nutritive. 



The ten elements previously considered essential could be 

 remembered by a mnemonic system suggested by Cyril Hopkins, 

 formerly of the University of Illinois, who used to tell his students 

 to think of "CHOPKNSCaFeMg," which, read as "C. Hopk'ns' 

 Cafe, mighty good" really contained the chemical symbols of 

 the ten elements needed by all the higher plants. Since we 

 have twelve in the list at the present time, it is here suggested 

 that if the reader thinks of "CHOPKNSCaFeMgBMn" and 

 reads it as "C. Hopk'ns' Cafe, mighty good, but mnemonic," 

 he can still remember the list without any great strain on his 

 memory. 



Calcium. — Calcium is obtained from the soil as either calcium 

 nitrate or sulphate. It is necessary in the plant for normal root 

 and leaf development. Plants with chlorophyll need more calcium 

 than those without and, in fact, fungi can develop in its absence. 

 Monotropa (Indian pipe), the saprophyte mentioned in Chapter 

 VII, has less calcium than normal autotrophic plants. The cell 

 wall contains calcium pectate and calcium is necessary for its 

 formation. Cells formed in its absence are thus likely to be weak 

 and easily injured, not only because of the absence of calcium in 

 the cell walls, but also because dividing cells in plants lacking 

 calcium contain large vacuoles, like old cells, and are thus defi- 

 cient in protoplasm. The cell divisions under such circumstances 

 are also very irregular and abnormal. Likewise root hairs do not 

 develop so profusely in the absence of calcium. Calcium favors 

 the digestion of starch and its translocation from one part of the 

 plant to another; and it may be for this reason that autophytes 

 (autotrophic plants) need more calcium than heterophytes. Many 



