8 PITH-KAY FLECKS IN WOOD. 



generally marked by a small black spot where the cambium is exposed 

 to the air. A number of puparia were secured in proximity to the 

 bark and always close to the termination of a mine. Pupation 

 occurred, as already stated, almost immediately after the larva 

 emerged. 



OCCLUSION OF THE LARVAL PASSAGES. 



Soon after the passage of the larva the healing process begins. 

 The larva destroys only those cells in its immediate path through the 

 inner bark. Cambial activity proceeds as usual on both sides of the 

 passage. As the cambium layer moves outward radialh^, the passage 

 left by the larva increases in diameter. For this reason, when 

 growth is very rapid, the pith-fleck spots are larger than when it is 

 slow. Mines produced in early spring are larger radially than those 

 made later in the season, when diameter increase is slower. 



The heaUng process proceeds mainly from the bast pith-ra}^ cells 

 in the bark which are in direct communication with the reserve food 

 supply. During this healing process made by the division of the 

 bark cells toward the wood, growth is proceeding in the cambium on 

 each side of the wound. The cambium becomes united again tovrard 

 the periphery of the annual ring, and wood elements are formed in 

 the usual way. Thus the cambium, reuniting around the passage, 

 leaves the latter behind in the annual ring. And the passage, 

 although thus wholh' independent of the new cambium, becomes 

 filled \Yiih new cellular tissue. This process can be followed macro- 

 scopically bv removing the bark from the infested trees. For a short 

 distance back of the larva the mine is free of all cell growth, and ap- 

 pears like a small groove under the bark. Soon, however, wound 

 tissue is formed, and the space is filled with young ceils similar in 

 character to those in pith rays. This tissue clings to the inner bark 

 when the latter is removed from the tree, and appears Kke a ridge 

 upon the smooth surface. After a distance of a few feet the wound 

 tissue gradually passes over into the wood or older portion of the 

 annual ring. The farther the mine is traced backward the deeper 

 is it buried, and in fast-grooving trees it often disappears completely 

 in the wood. 



Some idea of the microscopic appearance of pith flecks in trans- 

 verse section may be had from Plates II, III, and IV. The thick- 

 walled cells are occasionally separated by layers of a compact, yel- 

 lowish-broMTi substance, the remains of dead cells and excrement of 

 larvae. The size of the cells fiUing the wound does not depend upon 

 the size of the wound itself, but upon the nature of the tissue surround- 

 ing the wound; that is, upon the size of the ray cells and the diameter 

 of the vessels. Where the ray cells are compact and short, such as in 

 wild plum (PI. Ill, fig. 1), the successive divisions in the healmg 

 process give rise to cells of small dimensions. But when they are 



