Medullary Spots in Wood. 249 



section, until finally they break through the layer of cell frag- 

 ments (Fig. 6, c) and press like bubbles into the hollow space 

 (Fig. 6, d) where they grow very rapidly, divide in all directions 

 and soon fill the cavity. The ray cells of the bark were found to 

 be the most important and almost the exclusive regeneration 

 centers, though rarely other parenchymatous cells of the cortex 

 and very seldom ray cells of the xylem (Fig. 6, e) were observed 

 also to take part in the process. 



The dissolved cell fragments and larvae excrement are com- 

 pressed into a narrow border (Fig. 7, b) by the rapid growth and 

 division of the "filling cells." The pressure tends to keep back 

 other cells which otherwise would soon have proliferated into the 

 cavity. The "filling cells" assume very irregular shapes, the walls 

 thicken and grow darker (probably from tannin) and are densely 

 pitted with simple pits. Starch gradually makes its way into 

 these cells and often all the cells are found filled with starch 

 grains which contribute toward making the pith flecks resemble 

 the rays. (Fig. 7, c.) 



The cells which lie between the newly formed cambium and the 

 channel (Fig. 6, b), originally belonging to the bark but now 

 surrounded by the newly formed ring of wood, do not remain 

 unchanged, either; they thicken their walls appreciably, become 

 wood-like and in cross-section appear as wood cells though usually 

 more rounded. In longitudinal section they can be readily 

 recognized by their very short length in contrast with the normal 

 structure of the wood cells formed by the new cambium. 



Kienitz found pith flecks in the following woods: Alnus glu- 

 tinosa Gartn., A. incana D. C, Betiila puhescens Ehrh., B. ver- 

 rucosa Ehrb., Corylus avellana E., almost all species of Salix, 

 several species of Sorbus, Crataegus oxycantha L., C. monogyna 

 L., a few species of Prunus, probably also in some species of 

 Pyrus. He believed the somewhat similar spots in conifers, of 

 which Kraus^^ speaks, to be of different origin than those in 

 broadleaf species. 



A recent contribution to this subject has been made by Grossen- 

 bacher ^^. He made a careful study of the insect causing pith 

 flecks in current stems, and found that the mining was done by 

 the larvae of a tineid moth, Opostega nonstrigeUa Ch. (Fig i). 

 The larva is in many ways different from that described by 

 Kienitz, as may be seen by comparing Figs, i and 3, while the 



