FUNCTION AND STRUCTURE OF ROOTS 



123 



as in the pumpkin. When roots of this kind are thick and 

 fleshy, they are usually described 2.^ fascicled. 



172. The Two Modes of Growth - This difference in the 

 mode of growth is very apparent in the seedling, as will 

 be evident on referring to your sketches. The first kind 

 is called the axial mode, because it is a continuation of the 

 main axis of the plant ; the second is the 7ionaxial, or for 

 want of a better word we may call it the radial mode, since 

 the roots radiate in all directions from a common axis. 



173. Importance of this Distinction. — This distinction 

 has important bearings in agriculture. Roots of the first 

 kind, which are characteristic of most dicotyledons, strike 

 deep, and draw their nourishment from the lower strata of 

 the soil, while the radial kind spread out near the surface 

 and are more dependent upon external conditions. 



174. Root Structure. — Cut a cross section of any woody 

 taproot about halfway between the tip and the ground 

 level, examine it with a lens and sketch it. Label the 

 dark outer covering, epider- 

 mis, the soft layer just within 

 that, cortex, the hard, woody 

 axis that you find in the 

 center, vascular cylinder, and 

 the fine silvery lines that 

 radiate from the center to 

 the cortex, medullary rays 

 (in a very young root, these 

 will not appear). Cut a sec- 

 tion through a root that has 

 stood in red ink for about 

 three hours and note the 

 parts colored by the fluid. 

 What portion of the root, 

 should you judge from this, acts as a conductor of the 

 water absorbed from the ground.' 



Make a longitudinal section through the upper half of 



261. — Cross section of a young tap- 

 root : a, a, root hairs ; b, epidermis ; c, 

 cortical layer; d, fibrovascular cylinder. 

 Note the absence of medullary rays dur- 

 ing the first year of growth. 



