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, 

 <fec., be sown in a soil containing all the food-stuffs (K 2 0, P 2 5 , 

 &c.), except nitrogen, necessary for the growth of plants, then, 

 given sufficient moisture, germination will soon 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, albumin, 

 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 excep- 

 tions to be enumerated later persist in this state of starvation 

 and finally die away. Not so the Leguminosce. 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, throw- 

 ing up thick, juicy stalks, embellishing themselves with luxuriant 

 foliage, 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 nitrogenous 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 

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

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

 toes, &c. ; oilseeds, and so forth) that have been examined on this 

 point, are characterised by the capacity for growing and ripening 



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