134 



METABOLISM 



Series A, and to 27 mg. in C, being accounted for by the absorption of ammonia 

 in the gaseous form from the air. Concomitantly with this exclusion of combined 

 nitrogen there is a reduction in the amount of carbon and organic substance 

 formed, as well as in dry weight generally. It is worthy of note, however, 

 that an increase in dry weight may still occur, and that this increase is greater 



when plants are manured with the materials of the ash 

 than if they be grown in pure sand. The amount of nitro- 

 gen present in the seed reaches a greater amount than 

 can be accounted for by the limited supply in the ash. 

 The unequal development of plants treated in 

 different ways comes out even more prominently from 

 a study of Boussingault's figures than from a con- 

 sideration of the data quoted above. At Fig. 29, two 

 of Boussingault's figures have been reproduced, side 

 by side, reduced to the same scale ; i, represents a plant 

 from series B ; 2, from series A, although it might 

 stand equally well as a representative of series C, be- 

 tween which latter and series A the differences are not 

 worth mentioning. From the figures it will be ap- 

 parent that the normal plant may reach a height of 

 64-74 cm., and develop a prominent inflorescence,, 

 while that grown in absence of nitrogen reaches a 

 height of only 11-14 cm., and produced a capitulunr 

 of very limited size. 



This experiment shows with perfect clearness that 

 Helianthus is unable to make any use of the atmo- 

 spheric nitrogen. It also proves that potassium nitrate 

 forms a very appropriate source of nitrogen for nu- 

 tritive purposes, since the increase in dry weight in 

 plants belonging to series B is nearly sixty times that 

 of plants in series A. This great increase in dry weight 

 is very surprising, when one remembers how little 

 potassium nitrate the plants have been able to obtain. 

 A pot containing i^ kg. of sand received gradually 

 in the course of three months 1-4 g. of saltpetre, and 

 this amount was sufficient to enable two plants ta 

 reach their full normal development. 



Many hundred culture experiments in water and 

 sand have established the fact that nitric acid forms 

 Fi?. 29. Helianthus argo- zx\ excellent, not to Say the best possible source of 

 Jot£Tu^'n\tr;tl-%fwXoutu' nitrogcu for the great majority of plants. [How the 

 (Proportionally reduced.) After divergent rcsults arrived at by Treboux (1Q05) are 

 BoussiNGAULT, .860, pi. .. ^^ ^^ cxplaiucd it is, as yet, impossible to say.} 

 In principle it is immaterial with what base the nitric acid is united, still, 

 generally speaking, it is preferable to use such bases as are themselves essential, 

 hence potassium or calcium nitrates, although they are more expensive than 

 sodium nitrate, are most suitable in practice. It is impossible at present 

 to say whether nitrites as well as nitrates play any part in providing nitro- 

 genous nutriment to Phanerogams. According to Molisch's (1887) researches 

 these nitrites are very poisonous when present in high percentage, although in 

 dilute solution (0-05 per cent, or less in the case of potassium nitrite) they are 

 absorbed with avidity and undergo alteration in the plant ; strange to say they 

 are not oxidized into a nitrate, but, on the contrary, suffer reduction. Whether 

 a green plant can or cannot pass through all stages of its development when 

 supplied with nitrites only, is not known. 



