SECT. I 



MORPHOLOGY 



153 



section of such a root, in which the secondary growth has continued 

 for some years, can scarcely be distinguished from a cross-section of 

 a stem. By careful examination, however, the presence of primary 

 xylem in the centre of the root can be discovered, and its nature thus 

 determined. The wood of the root is also more porous than in the 

 stem, and bears a close resemblance to spring wood. On account of 

 this lack of differentiation in the wood, the annual rings of growth 

 are less distinctly defined in roots than in stems. 



In the root and its branches, as in the stem, the annual zones of 

 secondary growth become less numerous on approaching the growing 

 points (Fig. 150). 



Anomalous forms of Growth in Thickness (^^^). Extraordinary deviations from 

 the usual tyi^e of secondary growth are afforded by some stems and roots of Gynmo- 



2* » 



J.' u 



¥ic.. 161.— Transverse section of the stem of Mucuna altissima. 1, 2, 3, Successively formed 

 zones of wood ; 1*, 2* 3*, successively formed zones of bast. (J nat. size.) 



sperms and Dicotyledons. Among the Gymnosperms in the Cycadaceae and 

 certain species of G?ietuvi, in the Ghenopodiaceae, Amarautaceae, Nyctaginaceae, 

 Phytolaccaceae, and other families of Dicotyledons, the cambium which has been 

 formed in the ordinary manner soon loses its function, and a new cambium ring is 

 developed external to the bast zone, for the most part in the pericycle, or in a 

 tissue derived from it. This cambium ring forms wood on the inside and bast on 

 the outside, with the accompanying medullary rays. It then ceases to divide, and 

 a new ring takes its place. This process is repeated, and ultimately leads to the 

 formation of concentric rings of wood and bast, which, in cross-sections of the 

 sugar-beet, may be distinguished with the naked eye. These concentric zones may 

 be still more plainly seen in a cross-section of Mucuna altissima (Fig. 161), a liane 

 belonging to the order Papilionaceae. The stem shows in this case an inner axis 

 of wood (1) surrounded by a zone of bast (1*) ; next follows a cylinder of wood (2) 

 and bast (2*), and finally a third (3, 3*) in process of formation in the pericycle. 

 An extraordinary appearance is exhibited by cross-sections of stems, which show 

 several separate wood cylinders (Fig. 162). Such a structure is peculiar to various 

 tropical lianes of the genera S^ rjania and PauUinia belonging to the family 



