8o8 



A TEXTBOOK OF THEORETICAL BOTANY 



so that broad medullary rays mark the position of the primary xylems in 

 the old root. As growth continues centrifugally these rays may be gradually 

 narrowed, and in the outer zones of old roots they may be reduced to one 

 cell in width. This implies a gradual lateral extension of the broad wedge- 

 shaped masses of secondary tissues so that there is eventually a practically 

 continuous zone of secondary xylem and secondary phloem (Fig. 802). 



Cork 



Cork cambium 



Secondary 

 phloem 



Cambium 

 Medullary ray 



Primary xylem 



Secondary 

 xylem 



Fig. 802. — Ulmus procera. Transverse section of an older 

 triarch root showing secondary thickening. 



Exceptions sometimes occur in the roots of herbaceous or of climbing 

 plants, such as Ciicurbita, in which the cambium arcs never join into a con- 

 tinuous ring, with the result that the wedges of secondary tissue remain 

 distinct and the primary medullary rays are not closed (Fig. 803 and 804). 



Cambial growth ceases during the winter, though the cambial cells do 

 not disappear. When growth is resumed in the spring the discontinuity in 

 the growth of the wood is usually evident, so that annual rings may be seen 

 in older roots. 



The nature of the secondary wood varies greatly. In woody plants it is 

 usually compact and highly lignified, consisting chiefly of tracheids, with 

 small numbers of vessels and parenchyma cells, but in herbaceous plants it 

 may be very loose, groups of vessels and tracheids being mingled with 

 abundant parenchyma sometimes in a wholly irregular fashion. 



The elements in secondary wood, especially in the compact type, are 

 usually arranged in radial rows, corresponding to their origin from cells of 

 the cambial ring, an arrangement which distinguishes them from primary 

 xylem. Besides the large primary medullary rays there are many secondary 

 rays, one cell broad, which are formed in successive annual rings. New rays 

 are formed intermediate between previous rays, so that the average distance 

 between them remains approximately equal, in spite of the increasing 

 diameter of the root. 



