which contain the chloroplasts and thus constitute the site of photo- 

 synthetic activity within the needle. The mesophyll region is nearly 

 equally divided between these thin-walled cells and air spaces. Resin 

 canals are present near the lower surface of the needle on each side 

 of the midrib. 



The endodermis separates the mesophyll from the central peri- 

 cycle and fibrovascular bundle. The endodermal cells are large, 

 thickwalled and somewhat ovoid-elongate. Since the pericycle and 

 vascular bundle are not injured in any way by the fungus they are 

 not mentioned further. 



DISEASED NEEDLE ANATOMY 



After infection of a needle by R. pseudotsugae the ascospores 

 remained attached to the surface of the needle. The dispersion of 

 ascospores by rain water and moist winds apparently tended to 

 concentrate the relatively heavy ascospores on the upper surfaces. 

 Then too the smoother upper sides of needles, as opposed to the 

 jagged lower sides, seemed to afford a better surface for attachment 

 of spores. 



The black germ tube of the ascospore, and the hyaline extension 

 of the germ tube, penetrated directly through the cuticle regardless 

 of its thickness. No hyphae were evident in the stomata of needles. 

 Relatively thin hyphae penetrated the juncture between epidermal 

 cells and also grew between hypodermal cells rather than passing 

 through them. Upon reaching the mesophyll, the hyphae became 

 enlarged, septate and almost entirely intracellular. These hyphae 

 were not found until early September of 1956. By October the 

 hyphae had become branched, were globular in shape and appeared 

 to be filled with oil (Fig. 17). The hyphae varied from 3 to 5 \i in 

 diameter. Liese (29) stated that septation of such hyphae were 

 scarcely visible, but in the present study crosswalls were seen as 

 soon as hyphae were visible. 



Throughout the winter months, the hyphae penetrated further 

 into the needle, killed more cells, and thereby increased the size of 

 the visible lesion on the surface of the needle. Epidermal cells of 

 infected needles appeared to remain unchanged and did not differ 

 morphologically, nor react differently to stain, than did those of 

 healthy needles. The hyphae were largely of the coarse intracellular 

 type, but as the winter progressed, a few thin intercellular hyphae 

 were found throughout the needle. At times these penetrated the 



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