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A TEXT-BOOK OF BOTANY 



in diameter ( 24). The new wood, however, is not formed 

 in connection with the old wood, but just within the bast, 

 that is, farther in between the " spokes" of old wood, 

 resulting in bundles like those of the stem (Fig. 73). In 

 this way a thickening vascular cylinder is formed, like that 

 of stems that increase in diameter; and presently the cross- 

 section of the root resembles that of the stem. It is evident 

 (Fig. 73) that the principal pith rays separating the vas- 

 cular bundles of such a root extend inward to the original 

 radiating strands of wood that alternate 

 with the original strands of bast. The vas- 

 cular bundles of the root connect with those 

 of the stem, and these in turn with those of 

 the leaves, so that throughout the whole 

 plant there is a continuous vascular system. 

 The origin of the branches of roots is very 

 different from that of stems. In a stem 

 the branch begins at the outer part of the 

 cortex, but in the root it begins at the sur- 

 face of the vascular cylinder and breaks 

 through the cortex (Fig. 74). If the cor- 

 tex of a root be stripped off, the branches 

 will be found attached to the central axis, 

 and the perforations made by the branches 

 through the cortex can be seen. 



38. Growth in length. The elongating 

 region of the root is much more restricted 

 than that of the stem. It was stated ( 26) 

 that the elongating region of a stem may 

 extend ten to twenty inches from the tip, or even more; 

 but the elongating region of a root is hardly ever more than 

 two-fifths of an inch, and often not more than half of that. 

 The region of elongation and of greatest elongation should 

 be determined by using such seedlings as those of peas, 

 beans, and corn. When the young roots have become a 



FIG. 74. Longitu- 

 dinal section of 

 root of arrow- 

 leaf, showing the 

 branches start- 

 ing from the 

 central axis and 

 penetrating the 

 cortex. 



