MORPHOLOGY 



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the cambium produces medullary ray tissue, and this constitutes 

 the broadest medullary rays which lead to the strands of primary 

 xylem (Fig. 150, B), A cross-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 tissue 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 

 early 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. 



Anomalous forms of Growth in Thickness. — Extraordinary deviations from the 

 usual type of secondary growth are afforded by some stems and roots of Gymno- 

 sperms and Dicotyledons. Among the Gymnosperrus in the Cycadaceae and 



101. — Transverse section of the stem of Mucmia altissima. 1, 2, 3, Successively formed zones 

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



certain species of Gnetum, in the CAenopodiaceae, Amarantaceae, 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 » 

 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 repeats itself, and ultimately leads to the 

 formation of concentric wood and bast rings, 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. 151), 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 midst of 

 the pericycle. An extraordinary appearance is exhibited by cross-sections of stems, 

 which show several separate wood cylinders (Fig. 152). Such a structure is 

 peculiar to various tropical Lianes of the genera Serjania and Paullinia belonging 

 to the family Sapindaceae. This anomalous condition arises from the unusual 



