PHYSIOLOGICAL 287 



illumined by the investigations of Prof. Cathcart of Glasgow. But 

 let us now turn to plants. Have they any ashes from their hidden 

 fires, and if so, do they get rid of them ? The old view that plants, 

 being half asleep, do not form nitrogenous waste-products like urea 

 is contradicted by more careful biochemistry, for urea itself has 

 been demonstrated in some plants, and likewise the frequent occur- 

 rence of allied substances like asparagin, glutamin, and allantoin. 

 Of great interest also is the discovery of a urea-fermenting ferment 

 (urease) in many higher plants, as well as in many bacteria. Living 

 involves the breaking down of proteins, and this involves the 

 formation of ammonia. But ammonia, above a narrow limit of 

 quantity, is a cell-poison, so in the higher animals it is changed 

 into the relatively harmless urea or uric acid, while in some lower 

 animals it is neutralised in other ways. What happens in ordinary 

 plants is profoundly interesting and very different. For many, if 

 not most animals, it is profitable to get rid of the nitrogenous 

 waste-products (urea, etc.) as soon as possible; for fresh nitrogen 

 supplies are readily available in the food. But it is far otherwise 

 with ordinary green plants that have to find their nitrogenous 

 food sparsely in the form of nitrates (like saltpetre) and so forth 

 in the soil. That leguminous plants with their symbions are able 

 to tap the nitrogen supply in the atmosphere is no more to the 

 point at present than are other exceptional cases like insectivorous 

 plants and those that feed on decaying organic matter. An ordinary 

 animal is apt to suffer from nitrogen-excess, and man most of all; 

 but an ordinary plant is apt to suffer from nitrogen-deficiency. 

 This brings us to recognise the excellence of the plant's chemical 

 regime, for substances like asparagin and glutamin, which are 

 widely distributed, seem to be combinations which take the poison- 

 ous edge off the waste ammonia. In many a member of the fungoid 

 class the same role is discharged by urea itself. Thus plants and 

 animals meet in the mushroom ! 



But there is another adaptation besides drawing the ammonia's 

 teeth, for, as Kiesel and others have shown, the ammonia, rendered 

 harmless by combination, can be remobilised when nitrogenous 

 material is required by the plant in the building up of fresh protein 

 substances. This is the significance of ferments like urease and 

 asparaginase within the plant, that they re-liberate the ammonia 

 when it is needed. Thus the plant is more economical than the 

 animal, since it can use its waste-products as a reserve. It is plain 

 that this locking up of the ammonia, and its liberation again, must 

 involve a very strict regulation of the doors, but regulation is the 

 inmost secret of life. 



Finally, see how the plant cell's substantia] cellulose wall (and 

 so the tree's wood) is really an excretion of incompletely oxidised 

 material, done with so far as the protoplasmic metabolism is con- 



