H2 THE LIFE OF THE PLANT 



in its hairs, presents a large surface, although its volume 

 is insignificant, owing to the fact that this volume is 

 stretched out to so extraordinary a length. Here Nature 

 has had recourse to a trick similar to the one attributed 

 by poetic legend to Dido, the foundress of Carthage. 

 Dido obtained for the asking as much land as could 

 be measured by a single bullock's hide. As it turned out, 

 that same hide was made to enclose the whole future site 

 of Carthage, for Dido cut the hide into very narrow 

 strips. But the strips of Dido are not to be compared 

 with root hairs, which are considerably finer than a 

 human hair. 



It is most difficult adequately to realise the great 

 physiological importance of this prevalent development 

 in length. The root is thereby able at the smallest 

 possible expenditure of building material to encompass 

 the greatest possible number of particles of the soil, and 

 to come into the closest possible contact with them. 

 The data we possess are sufficient to calculate approxi- 

 mately the distance of the particles of the soil from the 

 surface of the root hairs of our wheat plant. 



For this purpose we must turn to the statistical 

 method, the procedure adopted by statisticians ; they, 

 neglecting the individuality of separate persons, speak 

 only of the average man, the average scale of living, 

 and so on, and describe phenomena in terms of average 

 quantities. Let us follow their example, and try to 

 depict in the most obvious way the volume of soil 

 occupied by an average root fibre of wheat. We know 

 the average area in a field occupied by each plant ; 

 we know the depth of soil occupied by the roots, and 

 hence know the volume of soil occupied by each plant. 

 This glass jar contains that measured quantity. Sup- 

 pose that we wished to transfer this soil to a vessel or 

 rather a tube one-third of a mile in length, i.e. of the 

 length of our root, what would the diameter of such a 

 tube be ? The answer works out to three millimetres. 



