340 Ward . — Recent Publications bearing on the 
as carrying away more nitrogen from the soil than could 
be accounted for. 
In illustration of this I may first give an abstract of a 
paper sent to me a short time ago by Professor Hellriegel, 
the Director of the Agricultural Experimental Station in 
Bernberg. 
Professor Hellriegers paper was published in November 
1 886, in the Zeitschrift des Vereins f. d. Rubenzucker In- 
dustrie des Deutschen Reichs 1 i and deals with the question 
of the sources of nitrogen in Gramineae and Leguminosae 
respectively. He was aided by Dr. Wilfarth. The author 
sums up the well-known points that, while nitric acid, am- 
monia, and certain complex organic compounds such as 
urea, uric acid, hippuric acid, proteids, and certain humous 
constituents, &c. are available as sources of nitrogen for 
plants, cyanogen and alkaloids and certain other complex 
organic compounds are useless for this purpose. Moreover, 
as proved by Boussingault (and he might have added by 
Lawes and Gilbert), the free nitrogen of the air is un- 
available 2 . 
It is also known that various natural processes lead on the 
one hand to the conversion of unavailable nitrogenous com- 
pounds into available forms, and vice versa ; and, on the other 
hand, to displace such compounds in the atmosphere and soil. 
For example, electric discharges, the evaporation of water, 
and the activity of certain micro-organisms aid in rendering 
nitrogen available, and rain, dew, and certain absorptive 
properties of the soil supplement or aid the processes. 
For a long time it has been generally known that the 
Leguminosae, especially, have what we may term a special 
aptitude for seizing large quantities of nitrogenous substances 
from the soil, and this property has become a classical puzzle 
in vegetable physiology. 
Hellriegel has been engaged for some time with this pro- 
1 ‘ Welche Stickstoffsquellen stehen der Pflanze zu Gebote,’ pp. 863-877. 
2 There is a short discussion of this subject in Dr. Vines’ ‘ Physiology of Plants,’ 
pp. 126-129. 
