8o8 



a parenchymatous ground tissue of considerable extent has been formed. 

 By cross-division of a number of cells which at first do not differ in form 

 from the ground mass and are but slightly elongated radially and longi- 

 tudinally, a number of meristematic centres are formed from which the 

 beginnings of thick-walled tissue elements start. By a very luxuriant 

 callus-like cell increase from the beginning, two parallel zones of meriste- 

 matic strands can be produced simultaneously, with the tissues as they 

 grow older. These parallel zones mature into two wood areas, which 

 remain distinct until they have become very thick. The formation of 



Fig. 189. 



I^ongitudinal section through the basal part of Fig 

 to be found from g to p. 



188, about in the zone 



isolated vascular bundles in the bark of our trees is not rare as is said to be 

 shown in tuber-gnarls. 



The first processes of change in the sapwood of the barked cherry tree 

 may be recognized in Fig. 189, which gives a longitudinal section from the 

 base of the barked portion in Fig. 188. H is the old wood, which because 

 of the cut has not changed any further, with its loosely reticulated vessels 

 {g). In the sapwood, lying just outside it, the cut has so affected the 

 nearly mature vessel {g) that its inner cavity has become filled with tyloses; 

 these have been used to form new cells and been changed into wood paren- 

 chyma. The new layer of wood parenchyma consists of only a few cells 



