THE ROOT 169 
HistoLoGy oF TRANSITIONAL GROWTH AND DEVELOPMENT OF 
SECONDARY STRUCTURE. 
At about six weeks of growth, one notes cells dividing by 
tangential walls in the inner curve of each phloem patch. This is 
intrafasctcular cambium. Its cells in each location start to cut off on 
their inner side a quantity of secondary xylem and add a little 
secondary phloem on their outer side. The protophloem tracts 
become pushed out and the protoxylem tracts in. The pith (if 
originally present) disappears as the protoxylem encroaches upon 
it. Secondary xylem finally fills up the patches between the 
arms. ‘The pericambium has a tendency to start division into an 
outer and an inner layer. The outer layer becomes a cork 
cambium (phellogen) surrounding the bundle inside of the endo- 
dermis. It cuts off cork tissue on its outer face, hence all liquid 
material is prevented from filtering through and cortex including 
endodermis, as well as the epidermis, shrivel, dry up and separate 
off at the age of two to three months. The cork cambium 
(phellogen) lays down secondary cortex internal to itself and external 
to the phloem. 
Patches of cells of the inner layer of pericambium divide rapidly 
and are called interfascicular cambium. ‘These join the intrafasctc- 
ular cambium to form a continuous cambium ring which then cuts 
off additional secondary xylem on its inner face and additional 
secondary phloem on its outer face pushing inward the first- 
formed or protoxylem and outward the first-formed or proto- 
phloem. Medullary-rays are formed by the cambium as it cuts 
off secondary xylem and secondary phloem elements. 
Thus, in a transverse section made through a portion of a 
Dicotyl root showing secondary growth, the following regions are 
noted passing from periphery to center: 
1. Cork 
2. Cork cambium (phellogen) 
3. Secondary cortex 
= 4, Protophloem 
% «\5. Secondary phloem 
z 2 6. Cambium 
5 = (7. Secondary xylem 
& 3\8. Protoxylem 
