476 



On the Structure of Roots. 



Fig. 5. 



nascent root Is pushed out through the seed-coats and takes a 

 downward direction of growth, forming thus a real descending 



axis, or inferior prolongation of 

 the stem. This course of de- 

 velopment may be very readily 

 traced in ordinary plants, such 

 as the mustard, turnip, carrot, 

 &c. A simple root of this kind 

 either maintains its original 

 character on a larger or more 

 complete scale, or its appear- 

 ance becomes eventually changed 

 by secondary growths. In the 

 common shepherd's - purse we 

 have an example of the main 

 root retaining its character as a 

 tap-root, moderately developed 

 and furnished with numerous 

 branches, all much inferior in 

 size. The carrot (fig. 5), parsnip, 

 turnip, radish, and beet, are 

 examples of roots of a similar 

 kind, which, when abundantly 

 nourished, are prone to lose their 

 ordinary woody character and 

 become swollen into large 

 masses of succulent material ; 

 these are tuberous tap - roots 

 (jfig. 6). When this is the case, 

 the main root is, as before, sup- 

 ported by a quantity of small 

 secondary branches, which, how- 

 ever, from their comparatively 

 small size, may be overlooked 

 by the superficial observer; the 

 lower portion of the root also, 

 for a considerable distance, re- 

 mains unaffected by the tuberous 

 growth, and displays a quantity 

 of fine fibrous branches which 

 contribute in large part to supply 

 the wants of the plant. The 

 tuberous development in the 

 plants just referred to, does not occur to any considerable extent 

 while the plants are in a state of nature ; they then produce only 

 rather thickened spindle-shaped fleshy roots. This fleshy con- 



Root of a seedling cirrot. 



