582 



the medullary rays have developed into wood cells with strongly refractive 

 walls, while the cell forms, to be found at a further distance from t\Vo 

 medullar}' rays, are still thinner-walled and richer in content, also showing 

 no broad ducts between them. In such sapwood layers, near the bud, a 

 tangential clefting of the tissue on the line between the wood of the pre- 

 vious year and that of the current year is found at times as the continu- 

 ation of the radial split. 



Radial holes (/) in the tissue of the secondary bark (>/). corres])ond 

 to tlie clefting of the wood body, while the j)rimary bark ( i)i ) with its bard 

 bast bundles (h) shows no ruptures whatever but only a partial browning 





^■y 



FiK. i; 



Internal .splitting- of a clion-y Viranrh produced by artificial tro.st. 



of the contents and of the walls of some of the hard bast cells and bark 

 parenchyma cells (r). Here also the holes are produced often by the 

 separation from one another of the different tissue complexes, less often 

 by the splitting of the membranes of the individual cells. The thin- 

 walled cell groups which, in the secondary bark, correspond to the bast 

 parenchyma of the primary Itark, separate from the bark rays which ha\e 

 already advanced further developmentally and are, therefore, thicker 

 walled. At the sides of these bark rays the rows of cells, accompanying 

 the hard bast cords and containing calcium oxalate, are especially noticeable. 

 The radial splits and clefts, however, are only secondary phenomena 

 in comparison with the great tangential clefts {s p) which separate the bark 



