CHAPTER III. ORGANS OF PLANTS. l8l 



Secondary Changes in Roots. Many roots, as those of 

 most Monocotyledons and vascular cryptogams, undergo little 

 change, except increase in size and the cutinization or lignifica- 

 tion of certain tissues in the course of their development ; while 

 others, as those of most Dicotyledons, may undergo very con- 

 siderable changes, either in the cortex, in the central cylinder, 

 or in both. 



In Gymnosperms, and some Dicotyledons, the primary cortex 

 disappears, and is replaced, largely, at least, by cork derived 

 from the pericambial layer ; in the roots of many other plants 

 the primary cortex persists, and cork is developed in its outer 

 layers, while new parenchyma is formed in the inner layers. 



The most important secondary changes, however, are those 

 which pertain to the central cylinder of the roots of Dicotyledons 

 and Gymnosperms. They consist in the formation of new wood 

 and liber by means of a cambium zone which, in appearance, 

 resembles that in the stems of the same plants, but which is 

 really of a different origin. 



It originates in the following manner : Behind or interior to 

 each phloem mass of the radial bundle that constitues the central 

 cylinder, cells begin to divide. These cells constitute an unin- 

 terrupted zone of meristem or cambium tissue. By its growth 

 the original phloem masses are carried outward, and form an 

 interrupted zone exterior to the xylem, while the outer cells of 

 the cambium develop into new phloem, and the inner ones into 

 xylem. In such roots, therefore, as in the stems of the same 

 plants, the phloem is exterior and the xylem interior, and these 

 are separated from each other by cambium. They also develop 

 medullary rays, and frequently rings of growth, but, except near 

 where the root joins the stem, they seldom possess a pith. Fig. 

 441 represents the two-rayed or diarch bundle of the young root 

 of the common Beet ; a is endodermis ; b, one of the phloem 

 masses ; c, cambium cells in the act of fission ; d, a duct of the 

 xylem. 



In some other cases, as, for example, in the roots of the 

 Pine, the pericambium that bounds the primary bundle becomes 

 a true cambium zone, and gives origin to new wood and new 

 liber. 



The roots of vascular cryptogams, except Lycopodiaceae, 



