SECONDARY THICKENING. NORMAL DICOTYLEDONS. 459 



piiate, according to the strict sense of the words. In the condition described under 

 (2) (a) and {b), on the other hand, the large medullary rays have originated secondarily 

 from the primary ones, primary rays in the sense of the first case having no more 

 existence after the completion of the ring of wood. The plants mentioned under 

 (2) {c) have, according to what has been said above, neither large nor primary 

 medullary rays ; those which appear later in the wood of Ephedra are all small ones 

 not reaching the pith. 



The cambial zone, finally, is divided into portions belonging to the strands and 

 to the medullary rays, into fascicular and interfascicular portions, according as it 

 borders on a medullary ray or on a strand of wood or bast. 



For Sanio's terminology, which differs from the above, and the employment of which 

 would seem to be attended with almost insuperable difficulties, comp. Bot. Ztg. 1863, 

 p. 372. 



In the cambial zone growth in the direction of the radii of the cross-section of 

 the stem goes on, with pauses in winter ; growth is followed by corresponding cell- 

 divisions ; of the products of division those bordering on the wood and bast are in 

 each case added to these as definitive tissue ; a zone lying between the two, however, 

 remains meristematic, and from this the process of new formation is repeated. 



The masses of tissue which are added by this process to the wood and bast are 

 the secondary wood, and the secondary bast. 



In the normal Dicotyledonous type, the differentiation of the two in the entire 

 secondary growth remains essentially the same as, or at any rate quite similar to, that 

 of the ring of bundles immediately after the first formation of the cambial zone and the 

 intermediate bundles. On the side of the wood, new elements, equivalent to the 

 first, are constantly added to the existing medullary rays, in their original direction, 

 in such a manner that in absolute dimensions and number of cells they either main- 

 tain everywhere the same height and breadth, or, as they extend in the radial 

 difection, they increase gradually, and usually relatively little, in breadth ; the latter 

 is the case especially in broad many-layered medullary rays, as in the stem of 

 Quercus, Casuarina, Clematis, Atragene, &c. The entire ligneous body accordingly 

 remains divided into the same number of main strands or main sections as that of the 

 strands existing between the large medullary rays, on the completion of the ring; and 

 these strands become successively broader towards the outside, being wedge-shaped 

 as seen in cross-section. On the one hand, they consist as before of elements 

 equivalent to their original ones, and the differences successively appearing in them 

 are to be described below ; on the other hand, radially-arranged plates of non- 

 equivalent tissue occur within the strands, which are essentially similar to the large 

 medullary rays in structure and direction : there are the small, short, secondary 

 medullary rays, which sever the main section or strand into partial sections. In 

 each successive zone of secondary growth new small medullary rays appear, each of 

 •which however, when once started, grows on in the radial direction, like the first 

 medullary rays. Every main section of the wood is therefore divided up by medul- 

 lary rays, which become successively more numerous and successively extend less 

 deeply towards the pith. 



On the cortical side completely similar conditions obtain to those in the wood ; 



