184 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY, 



its groups ready made. Schreiner and Skinner * have drawn up a 

 list of complex nitrogen compounds, which in their experiments 

 increased the amount of growth even in presence of nitrate. This 

 possibility is very interesting and requires considerably more 

 investigation. 



A much more subtle possibility has recently been discussed. 

 Animal physiologists have found that even a proper mixture of proteins, 

 along with purified fats, carbohydrates, and mineral substances, does 

 not constitute a perfect diet ; the animal makes no growth, but may 

 even develop diseases like beri-beri, unless a little milk or vegetable 

 juice is added. Until the nature of these compounds is known they 

 are designated by the non-committal title " accessory substances." 

 Again the interesting question arises : Do plants afford a parallel case ? 

 Armstrong has already put forward the suggestive hypothesis that 

 certain substances which he called " hormones " are advantageous 

 to the plant in altering the permeability of the protoplasm and thus 

 regulating certain vital processes, such as the intake of nutrient salts. 

 Some interesting facts are on record. We have found at Rothamsted, 

 for example, that cucumbers make better growth in water cultures 

 containing soil extracts than in cultures without soil extract even 

 when supplied with a complete nutrient solution. The experiment 

 is not easy to interpret because of the difficulty of analysing the 

 extract. But it would be attractive to think that some of the vague 

 physiological conditions that trouble the grower are to the plant 

 what beri-beri and similar diseases are to the animal — the result of 

 withholding some essential or useful " accessory substance." Bottom- 

 ley considers that certain substances obtained in the bacterial decom- 

 position of peat are of this nature. Still more recently Maze has 

 published some remarkable results showing that maize fails to grow in 

 water cultures containing all the recognized nutrient salts if these are 

 chemically pure, but it grows normally as soon as tap water is intro- 

 duced. No combination of added salts has as good an effect as the 

 tap water. 



Growth of Maize in Water Cultures. 

 (Maze, 191 5.) 



Culture solution (pure) -f Compounds of boron and arsenic . . 10 



-f „ boron and aluminium . . 24 



-J- ,, „ boron, aluminium, and arsenic i6'5 



,, + ,, boron, aluminium, arsenic, and 



iodine . . . . 366 

 „ (tap water) . . 43 



A considerable amount of work has obviously to be done before 

 the problem can even be clearly stated. 



But there seems to be no getting away from the fact that the 

 nutrition of the plant in the soil is mainly bound up with the decom- 

 position of the plant residues. In their original state these residues 



* U.S. Dep. of Agr., Bureau of Soils, Bull. No. 87, 1912. 



