44 ORIGIN OF [CH. Ill 



through the raot; and this in fact is their function, for 

 it is through these pipe-like vessels that the water 

 collected by the roots in the soil is transmitted to the 

 parts of the plant above ground. The absorption of water 

 by the root requires among other things that the root 

 shall present a large surface to the soil. It is only by the 

 extraordinary multiplication of surface that the plant is 

 able to perform what seems an impossibility : thus, if a 

 plant is kept for some time without water it is found, 

 just before it finally withers, to be obtaining water from 

 soil apparently as dry as dust. It is not usually realised 

 to what a depth and width roots extend; in a field of 

 winter wheat the roots have been found reaching to seven 

 feet beneath the surface, and in a single oat-plant it 

 was calculated that the length of the root including its 

 branches was 150 feet 1 . 



This manner in which roots branch has therefore some 

 importance. 



The roots which grow out from the primary root are 

 called secondary, these in their turn give off tertiary roots. 

 The first thing that strikes the observer is that it is only 

 on the older part of the root that secondary roots are seen. 

 Near the base (fig. 4), i.e. on the oldest part of the root, 

 are seen the longest, i.e. the oldest secondary roots ; and in 

 the region below they are shorter, while they are not to be 

 seen in the apical region. The figure also shows what is 

 characteristic of the secondary roots, namely, that they 

 are arranged in longitudinal rows, the roots in each ro\v 

 being accurately one above the other. This arrangement, 

 1 Johnson's 'How Crops Grow,' Dyer and Church's edit. 1869, p. 233. 



