432 Transactions. — Botany. 



sisted of pure quartz sand repeatedly washed, and was enriched 

 by a nutritive mixture that contained no nitrogenous com- 

 pound. The water used was specially prepared, and free from 

 ammonia and nitric acid. Moreover, the starvation of the 

 grasses grown under the same conditions proved the absence 

 of nitrogen compounds in the soil. It was thus quite clear 

 that the soil was not the source of supply of the accumulated 

 nitrogen of the leguminous plants. 



The only other possible source of supply is the nitrogen in 

 the atmosphere. Now, nitrogen exists in the air in two dis- 

 tinct forms — (1) As free nitrogen, which forms by far the 

 largest part of the air ; and (2) as combined nitrogen in the 

 forms of nitric acid and salts of ammonia (carbonate and 

 nitrate). Hence we must assume either that leguminous 

 plants have an extraordinary capacity for collecting and ab- 

 sorbing by their leaves the very scanty nitrogen compounds in 

 the air, or that they somehow make use of the abundant stores 

 of free nitrogen which it contains. 



The following considerations led Hellriegel to favour the 

 latter of these hypotheses : When peas are cultivated in a soil 

 free from nitrogen, and under the conditions described above, 

 two remarkabJy sharp periods of growth are to be noticed. 

 So long as the reserves of food stored in the seeds last, the 

 seedlings grow naturally, luxuriantly, and with the normal 

 colour. As soon as the reserves are exhausted, a somewhat 

 sudden change occurs — growth stops, the leaves turn pale, and 

 the plant evidently begins to starve. Sooner or later, how- 

 ever, the pale or yellow leaves again turn green, and a second 

 period of active growth sets in, after which the plants grow 

 normally to the end. When the arrest of growth sets in each 

 plant has about six leaves ; and, if these can fix and assimilate 

 the nitrogen compounds in the air, we are quite unable to 

 understand why the starvation phase should appear at all. If 

 the leaves cannot perform this function when in vigorous 

 growth and of normal green colour, hov/ can we suppose that 

 they begin to do so when in a sickly and discoloured con- 

 dition ? Hellriegel was not, however, satisfied with these 

 general considerations, but proceeded to carry out a series of 

 new experiments that would enable him to decide with cer- 

 tainty between the rival hypotheses. 



Pour vessels were filled with soil devoid of nitrogen, and 

 peas were put in and allowed to germinate. The vessels were 

 then placed under four bell-ja,rs completely closed, except 

 where they were joined by connecting tubes. The four vessels 

 were arranged in series, so that a constant stream of air was 

 drawn through from No. 1 to No. 4. Absorption vessels were 

 placed between each pair of bell-jars, and matters so adjusted 

 that the ordinary air passed into the No. 1 jar unaltered ; but 



