58 THE PLANT CELL. 



There is another very important consideration to be taken 

 into account in many of the vital processes which go on in a cell, 

 and that is the formation and action of those peculiar bodies 

 known as the enzymes (unorganised ferments). The chief 

 feature about these substances is the fact that \'ery small 

 quantities of them will produce very marked and extensive 

 chemical changes in other substances. Their action may be 

 expressed by the term catalytic, somewhat after the mode of 

 operation seen in the reaction between oxide of manganese and 

 ■chlorate of potash in the manufacture of oxygen. 



Of these enzymes the best known (in plants) are diastase, 

 which converts starch into dextrine and sugar,* and certain 

 peptic ferments which are present in the leaf-cells of such plants 

 as Drosera and Cariat papaya. The process which takes place 

 when an enzyme acts is known as hydrolysis. 



In the plant-cell, just as in certain animal cells (cells of glands), 

 the enzymes are probably formed by protoplasmic activity, a 

 precursor known as a zymogen, being first of all produced, and, 

 subsequently, by the action of water or an acid upon the 

 zymogen the enzyme is formed (see p. 1.59). The importance of 

 enzymes in a cell is undoubted, as upon their action depend most 

 of the chemical changes involved in the conversion of reserve 

 carbohydrate and proteid into forms more suited for direct use 

 by the cytoplasm.! The anabolic processes taking j)lace in a 

 cell are in many instances very complex, and it is only in a few 

 cases that distinct intermediate stages can be recognised, when 

 Buoh substances as starch and proteid are converted into proto- 

 plasm. Eecently the views concerning nitrogenous metabolism 

 have undergone a certain amount of revision, particularly when 

 it was shown that some plants {Bacteria) were able to utilise the 

 free nitrogen of the air, and convert it into substances which 

 were of further value to plants as sources of nitrogenous food- 

 material (see infra). A similar instance is that where filaments 

 of Beggiatoa are able to utilise sulphuretted hydrogen existing 

 in solution in natural springs, converting it into sulphur and 

 sulphuric acid by a process of oxidation. The katabolic side of 



* Some forms of diastase (oytase) can dissolve cellulose. 



t The action of enzymes increases up to an optimum, temperature 

 ranging from .30° to 45° C. Enzymes are destroyed at a temperature of 

 from 60° to 70° C. Darkness or subdued light appears to favour their 

 .action. 



