﻿Grossenbacher: Medullary spots and their cause 235 



Nielsen ior A. carbonaria and of ^. pruinosa by Greene also differs 

 from that of the wings of this species, as may be seen by comparing 

 a, b, and c of Fig, 5. However, Brown's observations, published 

 in the paper cited above, show beyond a doubt that the miner of 

 the river birch also passes the winter as a larva in the cambium of 

 the host, since on April 25 it was found feeding and measured 15 

 mm. in length. The Prunus miner appears to differ so materially 

 from the other species in both its life history and morphology 

 that I consider it best, at least provisionally, to name it Agromyza 

 Pruni sp. nov. 



The occlusion of the mines and the nature of the wound- 

 tissues produced 



The configuration of the mines was described in some detail in 

 my former paper but their occlusion and the reformation of the 

 cambium over the streak- wound were only incidentally mentioned. 



In sections showing mines near the cambium or within the un- 

 lignified zone one is able to see that a distinct new cambium de- 

 velops over the channels by the division of the first to the third 

 cell of the phloem outside the mine. Figs. 6 and 7 give some 

 idea of later but closely related stages. This new cambium is often 

 found formed over the wounds before the regular cambial line is 

 advanced that far, so that the cambium arches over the channels. 

 Fig. 8 illustrates this point. The first noticeable change taking 

 place in the cells surrounding a mine after the larva has passed is 

 the rapid growth of the uninjured cells on the radial and bark sides 

 of the channel. The phloem-ray cells usually enlarge more rapidly 

 than the others and give rise to bladder-like proliferations which, 

 after attaining considerable size, are cut off by septa from the cells 

 giving rise to them (Fig. 9) . The cells arising in this manner con- 

 tinue the growth in the same direction if space permits, and, if 

 other proliferations do not crowd on the sides, their diameter 

 becomes so great that septa soon form also parallel with the original 

 direction of growth. See Figs. 10 and 11. After the proliferating 

 outgrowths from the rays encounter the opposite side of the mine 

 the further enlargements are diverted in the directions of least 

 resistance that are most nearly in line with their former direction 

 of growth. In many instances of this type the cell back of the 



