CH. Ill] SECONDARY ROOTS. 45 



which gives a curiously formal, symmetrical look to a 

 branching root, depends on the fact that the secondary 

 roots spring from opposite the xylems, and since the 

 strands of xylem run straight down the primary roots, it 

 follows that the bases of the secondary roots run also in 

 vertical lines. The primary roots, as explained before, have 

 a quality of growth which enables them to grow straight 

 down; the secondary roots have a similar, but not an 

 identical, quality of growth, in consequence of which they 

 grow, roughly speaking, horizontally, or rather somewhat 

 obliquely. In this way they parcel out space between 

 them, the four secondary roots emerging at any given 

 level, run out north, south, east, and west. It is clear 

 that there will be unoccupied soil between the secondary 

 roots, especially when they have grown to some length ; 

 this space is taken up by the tertiary roots, and these are 

 not guided by any directive quality of growth in relation 

 to gravity but run out upwards, downwards, right and 

 ieft, thus making the most of the vacant places. 



One other characteristic of the growth of secondary 

 (and tertiary) roots must be described. In the fig. 4, at 

 the base of each secondary root, in the row facing the 

 observer, can be seen a vertical slit or cleft, through which 

 the root passes. This is explained by the mode of origin 

 of these organs, namely, that they arise in the pericycle. 

 That is to say, one or more cells in the pericycle of the 

 primary root begins to divide and form a mass of new 

 cells which constitute a very young secondary root. In 

 this stage, shown in fig. 18, it is obviously invisible from 

 the outside, since it is covered in by cortex. As it grows 



