CHAPTER V 

 THE MORPHOLOGY AND PHYSIOLOGY OF ROOTS 



1. The Distinctive Features of Roots 



Roots are typically underground parts which spread 

 through the soil and absorb therefrom the water and mineral 

 matters needed by plants, while simultaneously providing 

 a firm anchorage for the stems which rise in the air. Thus 

 roots have a distinctive primary with a prominent secondary 

 function. Though diverse in forms, and occasionally per- 

 forming additional or substitute functions, they are less 

 multiform in these features than leaves or stems, no doubt 

 because of the more homogeneous environment under which 

 they dwell. 



Typical soil roots extend from the base of the stem, and 

 continuously radiate, branch, and taper down to a fibrous 

 size. Taking all angles from vertical to horizontal, they form 

 collectively a mass suggestive of some shoots, but inverted 

 (Fig. 156). Unlike shoots, however, they are rarefy sym- 

 metrical, because mechanical irregularities in the ground, 

 and self-adjustments to the uneven distribution of water, 

 air, and mineral salts, greatly alter their shapes, making 

 actual root systems extremely irregular. The radiate form, 

 so distinctive of soil roots, enables them to reach a large 

 volume of soil, while also providing the best anchorage 

 against the all-sided strains to which stems are exposed ; but 

 there also occur cases in which a single main root continues 

 the stem vertically downward, the lateral roots being very 

 much smaller. Such a tap root (Fig. 157) is rare in trees 

 but common in herbs, as familiar in Dandelion and others, 



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