ENZYMES. 67 
first be reduced to glucose (grape-sugar), before they can be used as food by plants. 
When no invertase has been detected the general hypothesis has been that this 
inversion was due to the direct action of the protoplasm, but the recent isolation by 
Buchner and others of an invertase (Zymase) from yeast, in which it was long believed 
that none existed, once more emphasizes the uncertainty of negative conclusions. 
Diastase is common. _Is there more than one kind, z. ¢., a sort which can only 
convert the starch into amylodextrin and another which converts it into maltose 
and dextrine? In many cases, when the organism ts grown on potato, the con- 
version is carried only a little way and stops, there being always a copious purple 
or red-purple reaction with iodine. In other cases, e. g., when Bacterium campestre 
is grown on potato, the starch conversion is so complete that after a few weeks there 
is little or no color reaction when the potato-cylinder is mashed up and iodine water 
added. What makes this difference? 
A substance capable of dissolving the middle 
lamella appears to be common to all bacterial plant 
parasites and a true cytase presumably occurs, but 
much additional study is necessary. Probably 
several enzymes are confused under this name, 
just as several chemically different substances are 
still called “cellulose.” The substance which 
dissolves the middle lamella in some cases is prob- 
ably ammonium oxalate. The writer has not been 
able to dissolve it by means of pure oxalic acid, 
but that of turnips softens in ammonium oxalate. 
The lab or rennet ferment is rather common. 
Its action should not be confused with the curdling 
of milk due to the formation of acids. Tests may 
be made in litmus milk. Is there more than one 
kind of such ferment? Some organisms coagu- 
late the milk promptly into a solid mass which 
finally shrinks, extruding whey. Others cause the 
Fig. 58,* casein to separate out of the fluid very slowly as a 
multitude of separate particles which only become compacted very slowly. 
The writer has not met with the oxidizing enzymes, unless the substance in 
bacterial cultures which causes rapid evolution of oxygen from hydrogen peroxide is 
such an enzyme, as Dr. Loew maintains (Bibliog., XLV). Many other enzymes 
undoubtedly occur and play their part. The student should search for emulsin, 
lipase, lactase, maltase (glucase), etc. 
All known enzymes when freely exposed to steam heat are destroyed at tempera- 
tures considerably under 100° C. ‘They are less sensitive to heat than the bacteria 
themselves, but are destroyed by a few minutes exposure to temperatures 15° to 30° C. 
(moist heat) above the thermal death-point of the organisms which have produced 
*Fic, 58.—Thick-walled Kitasato flask for filtration or evaporation in vacuo, etc. Much re- 
duced, 
