98 COMPENDIUM OF GENERAL BOTANY. 



the bnndle-sheatli lias actually been observed. Should the central 

 portion be wantmg, it would be difficult to decide definitely whether 

 the given wood belonged to a root or to a stem (a difficulty encoun- 

 tered by palaeontologists). 



Fleshy roots ' (turnips) owe their condition, as a rule, either to 

 the prevalence of the parenchyma (longitudinal parenchyma and 

 ray-parenchyma) in the woody body or to the extensive develop- 

 ment of the secondary cortex. 



" Abnormal" root-structure is to be considered from a stand- 

 point analogous to that of the stems referred to above. 



(Ji) Anatomy of the TroMsition-zone hetween the Stem o/nd 



the Root. 



The question as to how the existing differences between the 

 root and the stem become equalized at the transition-zone must 

 force itself upon every anatomist who has studied the stem and 

 root. Evidently the elements which conduct water in the roots 

 continue the same function in the stem ; hence it must undergo its 

 typical transformation into stem-structure at a region near tlie sur- 

 face of the soil. The description given by de Bary' (after 

 Strasbruger) of the case of Biota orientalis is simple and explicit. 

 The student can easily study the transition-zone in longitudinal and 

 cross sections. 



The hypocotyledonous stem contains two bundles in its upper 



part, which extend perpendicularly downward from 



the two cotyledons; the bundle of the main root is 



diametrically diarch. In each of the two cotyledonous 



bundles the phloem (leptome) divides into two equal 



parts near the base of the cotyledons. The leptome- 



halves diverge more and more, and are finally in the 



sanje tangential plane with the hadrome-group. Each 



one of the sieve-tube groups approaches a correspond- 



FiG. 59. -j^g group from the other bundle and unites with it 



to form a broad leptome-group. The elements of both portions 



of the vascular system (leptome and hadrome) undergo a similar 



displacement or torsion at tlie same level of their course ; so that 



' Weiss, J. E., Flora, 1880 ; also other authors. 



' Comp. Anatomy, page 386. Also Dodel, Pringsheim's Jahrbticher, VIII. 



