THE ROOT 107 



(fig. 31, odd numbers) grown in the soil that contained 

 the bacteria which have formed nodules, and have 

 developed normally; the others (fig. 31, even num- 

 bers) have perished. This proves that the property 

 which differentiates peas and likewise all leguminous 

 plants from cereals, namely the property of assimilat- 

 ing free nitrogen from the air, is bound up with the 

 capacity of their roots for becoming infected by certain 

 soil-bacteria. How and where this assimilation of 

 nitrogen takes place has not yet, however, been fully 

 explained. 



In view of the fact mentioned above that the soil- 

 water is such an extremely weak solution of nutrient 

 salts, it may well be asked whether it is really sufficient 

 for the nutrition of plants ? We can answer this 

 question by reference to the following calculations. 

 We know the quantity of rain that falls on a certain 

 area of land ; we also know the quantity of nutrient 

 substances that this rain water can extract from the 

 soil ; on the other hand we know the quantity of ash 

 contained in a whole crop, gathered from the same 

 area of land. These data are sufficient to provide an 

 answer to our question whether this liquid food is 

 sufficient. It is only the most fertile soils for which 

 the answer is in the affirmative, in all other cases 

 the answer is negative. In general the liquid food 

 alone is not sufficient for the plant. Evidently it must 

 also use substances insoluble in the soil-water. But 

 in that case the root must, so to speak, seek out its 

 own food, searching round all the neighbouring particles 

 of the soil in order to find among the mass of sterile 

 matter minute particles of the nutrient substances so 

 sparsely scattered in it. This brings us directly to 

 the consideration of the second question we have 

 raised. Having ascertained wherein the food of the 

 root consists, let us now try to find out how it gets it. 



