8oo 



The new structure begins with the appearance of a layer of cork cells 

 at the periphery of the healthy medullary ray tissue, cutting off an outer, 

 dead part. The living part of the medullary ray now develops several 

 layers of parenchymatous cells about its circumference, which cells turn 

 green like the medullary tissue already present. By the increase of the 

 parenchymatous tissue around the medullary rays, a callus roll is produced, 

 which rapidly becomes larger and constantly presses farther outward the 

 cork layer which begins with the formation of lenticels. "The new cell 

 tissue does not develop on any one place from the living medullary ray, but 

 as everywhere new cells are formed in all places inside the cells already 

 formed ; these reabsorb the mother cell, grow out to its size and widen the 

 mass on all sides. In spite of the widening of the callus, due to the growing 

 cell tissue, the living part of the medullary ray, nevertheless, always retains 

 the same circumference, the same size, number, form and position of the cell 

 tissue constituting it." 



"When the callus reaches a certain size, different parts become unusu- 

 ally thick walled, as is also the case in the normal course of the life of 

 the bark (stone cell aggregations). Further, on each side of the living 

 medullary ray not far from its tip, a vascular bundle develops in the cell 

 tissue, which consists of pitted tracheids and vessels between the medullary 

 ray and the cork layer." By the fusion of the individual coordinate tissue 

 zones of the new structures, which up to that time had been completely 

 isolated and wart-like, a continuous bark layer covered with a cork layer is 

 produced, differing only by the radial arrangement of its cell elements in 

 cross section from the structure of the normal bark. "Along the sides of 

 the tip of the medullary ray, the development of the wood advances up to 

 the formation of a connected wood layer, traversed by the cell tissue of the 

 old medullary rays just as by newly formed, smaller ones. The various 

 wood bundles consist of tracheids and fibres. True spiral elements are 

 lacking. A line of division between the wood and the bark (Meristem zone 

 Ref.) is formed more and more sharply with the advancing development of 

 the wood, although no trace can be discovered either of phloem fibres or 

 of sieve tubes." 



Th. Hartig's observations, which represent an important advance, 

 show, therefore, that the development of the new bark on a bark injury, 

 takes place at the expense of the nutritive substances present in the wood 

 and begins with the formation of a callus tissue around the tips of the 

 medullary rays. 



It cannot be learned either from the description, or from the drawings, 

 which cells initiate the callus formation. 



TrecuP fills this gap with his thorough anatomical investigations, which 

 prove at the same time the participation of the zvhole young tissue left on 



1 Trecul, Accroissement des vegetaux dicotyledones ligneux. Annales des 

 science, nat. XIX, p. 165. 



