TISSUES AND SIMPLE ORGANS. 97 



maiy vessels) are somewhat scattered through the root, as seen in 

 cross-section. 



Ad 2 and 3. The number of vascular bundle-groups (hadrome- 

 bundles) corresponds to the number of leptome-bundles, and ac- 

 cording to this numerical relation roots are designated as ' ' di- 

 arch," "triarch," "tetrarch," etc., " polyarch." The latter, 

 that is, many bundle-groups, occur most frequently among mono- 

 cotyledons ; the former (" diarch, " " triarch, ' ' etc.) are more com- 

 mon among dicotyledons. This anatomical relation is perhaps the 

 reason why secondary growth in thickness cannot take place in the 

 roots of monocotyledons, since the numerous primordial bundles re- 

 quire eo ipso a maximum circumference for their maturation. 

 Among dicotyledonous roots secondary growth in thickness oc- 

 curs frequently. As is well known, conifers and other trees often 

 have roots a foot or more in diameter. 



How do such roots grow in thickness ? Fig. 58 represents 

 the period at which secondary growth in thickness begins. The 

 meaning of the letters are clear, with the exception of -y, which in- 

 dicates the cambial tissue. Tangential walls are formed within 

 the sieve- tube bundles; cell- divisions continue along the sides of 

 the bundles ; finally, this process also begins immediately outside of 

 the primordial vessels, upon which the cambial ring is completely 

 closed. During the earlier stages this ring is two, three, or more 

 lobed or loculose ; later, when the secondary wood-formation has 

 begun within the primordial albumen-bearing tissue, it becomes 

 circular. In general, this cambium behaves like that of the stem, 

 forming woody elements (wood-parenchyma, medullary rays, vessels 

 and tracheids) inwardly, leptome and ultimately mechanical bast-cells 

 outwardly. Thus, finally, the root can only be distinguished from 

 the stem by the structure of the organic centre ; in the root this 

 must show a central tissue of three or more radiating primordial ves- 

 sels instead of the medulla. Later the primordial leptome of the 

 numerous conducting bundles is no longer found in the centre ; it 

 is crowded toward the periphery by the formation of the woody 

 tissue. According to the above, there are certain portions of the 

 pericambium p which take part in the formation of the cambium- 

 ring, namely, those cells which lie above the vessels first formed. 

 The bundle-sheath s (protective sheath) must either grow with the 

 increase in thickness of the bundle or rupture. Such growth in 



