[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 329 



and margins. Terminal bud dies. Roots short, much branched, dark brown in 

 color. 



Boron deficiency. Young leaves in terminal bud at first light green at base; 

 later disintegration is evident, followed by twisted growth. Terminal bud dies. 

 Roots have many short laterals; brown in color. 



If one were to attempt a complete explanation of each deficiency 

 effect, it would be necessary to know the first effect of the chemical ele- 

 ment on one or more processes in the cell and then to show that without 

 these first effects certain other processes would not occur. This is not 

 easy to do, and with our present knowledge it is usually impossible. 

 Perhaps the reader would be content to say that chlorosis in the absence 

 of magnesium is the result of the fact that magnesium occurs in the 

 chlorophyll molecule and no chlorophyll could be made without it. But 

 how shall we explain chlorosis in the absence of iron or of manganese? 

 We cannot, of course, pursue this problem in detail in a general textbook, 

 but we may emphasize certain useful perspectives. 



In previous chapters each of the major plant processes (photosynthe- 

 sis, respiration, digestion, assimilation, etc.) was presented in a simple 

 manner. That is, the products used and the products formed were 

 named, and it was usually noted whether oxidation, reduction, conden- 

 sation, or hydrolysis was involved. We omitted the series of intermediate 

 steps in each of these major processes, partly because many of them 

 have not yet been discovered, and also because they involve many com- 

 plex chemical compounds. For example, in muscles of animals, glycogen 

 is changed to lactic acid, and vice versa. It is now fairly well established 

 that at least 10 diflFerent enzymes and as many kinds of intermediate 

 products are foiTued during these major changes. Furthermore, nearly 

 every one of these enzymes and intermediate products is adequately 

 active only when phosphorus is present and temporarily combines with 

 them; that is, phosphorus is a catalyst in these processes. It appears to 

 be a catalyst in most of the major processes in living cells, and for that 

 reason has been referred to as "the dynamite of living cells." Many other 

 elements, such as iron, manganese, magnesium, and potassium, are also 

 catalysts in some of the major processes that occur in cells. 



From the above discussion it is obvious that one cannot understand 

 all the relations of the mineral elements to cell processes until all the 

 steps in each of these processes are fully known. We may, however, at 

 this time summarize the relations of the elements already discussed in 

 previous chapters. We have seen, for instance, that nitrogen, sulfur, and 



