﻿238 Grossenbacher: Medullary spots and their cause 



as soon as those of the normal tissue at an equal distance from the 

 cambium; they also show that the cells making up medullary spots 

 are not all of the same age nor produced under equal and like pres- 

 sures: some of them lignify before others, some are angular while 

 others are nearly circular in cross section. Figs. 9, 13, and 14 show 

 practically all of the types usually found. 



The study of the development of such substitute tissues has 

 much of indirect interest to plant pathology, and the wounds, 

 when numerous, may sometimes not only prove very harmful to 

 the host but incidentally afford entrance to parasitic organisms. 

 In the case of Pruniis no such connection with a fungous disease 

 was established ; a very striking example, however, was found in con- 

 nection with the study of the Rihes cambium miner discussed in 

 my earlier paper. In that case many of the shoots, vacated by a 

 miner that it might pupate in the ground, were invaded by a fungus 

 through the exit hole and killed back. 



Description of plates 10 and XX 



Upper right collected May 30; upper left collected June 4; lower right collected 



Fig. 3. Female with ovipositor extruded. From b 



from lenticel above, and the lower one upward from ler 



Nielsen; b. Wing of A. pruinosa, from Greene; c, Pho 



Fig. 6. Medullary spot in Pruti 

 stage in the development of a new ca 



Fig. 7. Medullary spot in Prum 



Fig. 8. Medullary spot 



Fig. 9. Medullary spot in wh 

 variation in the size and shape of 



Fig. 10. Medullary spot. 1 

 the rays and by the incurving of the r 

 eral growth on the bark side. (Upper, 



Fig. II. Medullary spot, made b 

 ray on bark side and by bulging of ray fr 

 ray are shown also to have undergone longi 



