TERTIARY FORMATIONS IX THE ROOT. 115 



348. The distinction of texture marking the periods of rest 

 is not clear in the liber, though even here it may sometimes be 

 detected. The cork of the root frequently exhibits such dis- 

 tinction, but never so clearly as does the cork of stems. 



349. It is a familiar fact, that the fleshy roots of many plants - 

 beets, and the like exhibit in the first year from seed concentric 



/ 



rings, which resemble those found in perennials. This appear- 

 ance is due, according to de Bary, 1 to the fact that at an earl}' 

 stage of development (when the root is only about half a milli- 

 meter thick) a new cambium zone is formed in the parenchyma 

 on the outer part of the central cylinder, and this divides tan- 

 gentially, extending therefore in a radial direction, producing 

 woody and liber elements, and at the same time divides laterally, 

 so that the whole constitutes a zone hardly broken by the rays. 



/ m/ m/ 



Soon a second zone is produced in like manner, and afterwards 

 others. In all these cases the elements are usually not much 

 lignified, but. the whole mass remains succulent. 



It happens sometimes that tertiary formations are produced in 

 the root, bearing somewhat the same relation to the secondary 

 as these do to the primary. Even formations of higher order 

 are sometimes met with. But the elements of all of these are 

 easily identified, and their mutual relations can generally be so 

 clearh' understood that they do not need special description. 

 The following enumeration embraces the most important of these 

 formations: tertiary cork and cortex; fibro-vascular bundles in 

 secondary cortex ; tertiary liber and wood in secondary wood. 

 Such anomalies are more frequent in the stem. 



350. Roots branch by the development of certain cells at the 

 peripheral layer of the central cylinder, and just in front of the 

 woody fascicles. 2 



' 



The root branches only laterally in flowering plants ; in the 

 Lycopodiacese there appears to be terminal bifurcation, and here 

 each branch shares with its fellow the tissue elements of the root 

 from which they both come. 



1 Vergleichende Anatomic, p. 616. 



2 Three types of branching are described by Janczewski : 1. The mother- 

 cells of this layer (the so-called Rhizogenic cells) most frequently give rise 

 to all the tissues of the rootlet. 2. They produce only the central cylinder 

 and cortex, but not the root-cap and piliferous layer, these being furnished by 

 the endodermis of the root. 3. They produce only the central cylinder, the 

 other tissues coming from the endodermis or from the layers immediately out- 

 side of it. The subsequent growth of the rootlet both in length and thickness 

 is like that of the root. 



