DECAY 187 



imply that there is some special property in the ray itself which 

 favors a more vigorous growth there, and serves to retain the 

 fungus in that particular locality. The terminal walls, as in 

 Abies, Larix, Picea, etc., are perforated with numerous pits, which 

 would offer a somewhat easy path for the radial extension of the 

 fungus. But such openings fail to satisfy the conditions and 

 explain the great abundance of mycelium found in the rays, 

 since the terminal walls of the Cupressinese are not pitted but 

 present a blank wall to the further progress of the fungus. 

 Furthermore it may be shown, as will appear very shortly, that 

 physical openings offer no determining influence upon the direc- 

 tion of growth of the mycelium, which continues in the originally 

 selected course without respect to the structural characteristics 

 of that which may lie in its path. From these facts, then, it 

 would seem that the structure of the ray does not afford an 

 adequate explanation of the observed phenomena. 



The medullary ray constitutes perhaps the most important 

 structural region within the vascular cylinder with respect to 

 the accumulation of reserve food. This is deposited in the form 

 of starch and other easily assimilated products, and it is their 

 presence in relative excess which undoubtedly determines the 

 abundant development and localization of the mycelia within 

 such regions in the first instance. If this hypothesis be regarded 

 as a correct one, then it is possible to see how the mycelia gain 

 access to other structures exactly in accordance with its require- 

 ments and the possibilities of finding fresh stores of food mate- 

 rial, which may be held to appear in diminishing quantities as 

 successive areas are entered, until, the more available forms of 

 food having been exhausted, the cellulose fabric itself is attacked, 

 and with its disintegration the characteristic features in the 

 operation of the fungus are expressed in recognizable form. In 

 confirmation of the view thus expressed, it will be found to be 

 very generally true that next to the medullary ray the greatest 

 development of the mycelium takes place in the tracheids and 

 resin passages, which they traverse in a longitudinal direction 

 (plate 9). In this case it is possible that the opportunities for 



