8o9 



and exhibits immediately the first stages of thicker walled elements in the 

 form of shorter, porous vessels (gs) as the first production of the newly 

 formed cambial layer c-c. Each successive tissue layer, formed from the 

 cambium, has longer vessels ; at h, we find thin-walled elements, shortened, 

 to be sure, but unmistakably resembling the normal wood cells ; correspond- 

 ing to these thin-walled elements, the phloem elements appear at j in the 

 bark (r) : ;ir is the xylem ray, ph the phloem ray. 



If in the early spring, when the bark is easily loosened, homologous 

 cells are torn around the whole circumference of the trunk in the process of 

 removing the bark, thereby causing a reproduction of similar bark, arising 

 from similar elements, we find that the bark wounds become more irregular 

 from the time of the leaf development until late in June. More cell groups 

 remain attached to one place on the wood cylinder than to another and the 

 new structures differ accordingly. It thus happens that pieces of sapwood 

 of the current year, which contain vessels, are forced up by a callus tissue 

 produced beneath them. 



If the bark wounds are left uncovered, the new formation of bark will 

 in many cases be more doubtful. According to my experience, the bark 

 regeneration succeeds better in July, for some trees in August, than in April, 

 May or June. The maple and alder must be barked earlier; numerous 

 experiments made on these trees in August gave no results at all. 



If the bark wound, made in the heat of the day and left without any 

 protection whatever, is investigated after some hours (sweet cherry trees 

 were used for the experiment) it was found that the color of the originally 

 white wood cylinder had changed to yellow. The wound surface owed this 

 color especially to the browning of the medullary ray cells. 



The browning is more intense on the southwest side than on the north 

 side. 



The medullary rays are easily recognized by the fact that immediately 

 after the removal of the bark they project somewhat above the barked 

 surface. 



This fact indicates that the medullary ray cells at the same radial dis- 

 tance from the median line of the trunk have firmer walls than the young 

 wood cells, i. e. their development is further advanced than that of the 

 equally old cells of the vascular bundle. 



Such an advance of the medullary rays over the other tissue will stamp 

 them as a tissue of increase, which creates space for the newly produced 

 ivood tissue in the direction of the radius of the trunk. 



This prominence of the medullary ray groups takes place also in part 

 because of the more rapid outcurving of their outer walls, resulting, as a 

 rule, from the barking. These outer walls (unprotected) grow thick very 

 rapidly and turn brown. 



The cell contents increase in the medullary ray and young wood cells 

 lying immediately beneath the wound surface; masses of cyptoplasm and 

 later of starch appear, the former, when treated with glycerin, rounds up 



