CHAPTER XXXIII. 



THE FIXATION OF FREE NITROGEN BY BACTERIA. 



191. Accumulators and Consumers of Nitrogen. 



IF seeds of any of the leguminous plants, e.g. peas, lupins, clover, &c., be 

 sown in a soil containing all the food-stuffs (K 2 O P 2 O, ( , &c.), except nitrogen, 

 necessary for the growth of plants, then, given sufficient moisture, germina- 

 tion will scon be observed. At the outset the young plant develops just as 

 well in the absence of nitrogen as if that substance were present in the soil : 

 it feeds upon the stores of nutrient substances (carbohydrates, albumen, fat, 

 &c.) accumulated in the cotyledons or seed-leaves. When, however, this store 

 is exhausted, a complete cessation in the growth of the plant visibly ensues, 

 the leaves turning yellow and becoming partly dried up, and the whole plant 

 presenting a moribund appearance. The cause of this condition can only be 

 sought in the dearth of combined nitrogen that has now set in, since the 

 plant has all the other essential food-stuffs at its disposal. The condition 

 itself is consequently termed nitrogen-hunger. All other kinds of higher 

 plants hitherto examined suffer in the same way when grown under these 

 identical conditions. Differences, however, are noticeable in their subsequent 

 behaviour. If left unprovided with assimilable nitrogen, the representatives 

 of all other families of phanerogamic plants apart from a few exceptions to 

 be enumerated later persist in this state of starvation and finally die away. 

 Not so the LeguminoscB. These will be observed to remain for some time 

 a few days to three weeks, according to circumstances in this debilitated 

 condition, but will then revive almost instantaneously, rapidly turning green, 

 throwing up thick, juicy stalks, embellishing themselves with luxuriant foli- 

 age, putting forth a large number of blooms, and producing a good crop of seed. 

 Such a plant may then overtake others that have been provided with nitro- 

 genous food (manure), and have not had to pass through the famine period ; 

 and may, finally, at harvest yield a quantity of haulm and seed as great, 

 and containing just as much nitrogen, as its more highly favoured fellows. 

 Hence the legurninosse, in contrast to (nearly) all the other cultivated plants 

 (cereals, such as wheat, oats, &c. ; hoed crops, such as beet, potatoes, &c. ; oil- 

 seeds, and so forth) that have been examined on this point, are characterised 

 by the capacity for growing and ripening in a soil perfectly devoid of nitro- 

 gen and without the application of nitrogenous manure. This circumstance 

 is so much the more remarkable since both the leaves and seed of pulse 

 contain an unusually large proportion of combined nitrogen, and are, in fact, 

 richer in this element than any other vegetable food-stuffs. This fact will be 

 best displayed by the subjoined table, giving the average figures, obtained 

 from a large number of analyses, of the percentage of nitrogen in the dry 

 matter of the seeds of 



Maize 1.8 



Buckwheat . . . .1.9 

 Oats . . . . .1.9 

 Wheat 2.3 



Peas 4.3 



Beans 4.6 



Lentils .... 4.7 

 Soja beans . . . .6.1 



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