MORPHOLOGY 



143 



sperms and Dicotyledons. Among the Gymnosperms in the Cycadaceae and 

 certain species of Gnetum, in tlie Chenopodiaceae, 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 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 dist nguished with the naked eye. These concentric zones may be 

 still more plainly seen in a cross-section of Mucuna altissima (Fig. 152), a liane 

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

 of wood (1) suirounded 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. 



Ki<;. Iji'. --Tr.-insviTsi' section of the stem of Mitcuiui altiasima. 1, -2, 'A. Successively formed 

 xi nit's of wood ; 1*, 2*, 3*, successively formed zones of bast. (] nat. si/p.) 



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

 several separate wood cylinders (Fig. 153). Such a structure is peculiar to various 

 tropical liancs of the genera Serjania and Paullinia belonging to the family 

 Sapindaceae. This anomalous condition arises from the unusual position of the 

 primary vascular bundles, which are not arranged in a circle but form a deeply 

 lobed ring ; so that, by the development of inter fascicular cambium, the cambium 

 of each lobe is united into a separate cambium ring. Each of these rings, in- 

 dependently of the others, then gives rise to wood and bast (Fig. 153). A very 

 peculiar structure is exhibited by many lianes of the Bignoniaceae, the wood of 

 which is cleft by radially projecting masses of bast (Fig. 154). The primary stem 

 of the Bignoniaceae shows the ordinary circular arrangement of the vascular 

 bundles. Wood and bast are at first produced from the cambium ring in the usual 

 manner, and an inner, normal wood cylinder of AXIAL WOOD is formed. Such normally 

 formed axial wood cylinders are common to many, otherwise abnormally developed 

 lianes. The cambium ring of the Bignoniaceae, after performing for a time its 

 normal functions, begins, at certain points, to give off internally only a very small 

 quantity of wood, and externally a correspondingly large amount of bast. As a 

 result of this, deep wedges of irregularly widening bast project into the outer 



