1 88 



ANATOMY OF THE GYMNOSPERMS 



free growth afforded by the long cavity of the <tracheid or resin 

 passage may serve somewhat to influence the direction of growth 

 as the fungus searches for food, although, as in the previous 

 case, it cannot be regarded as a determining factor of primary 

 importance, inasmuch as there is a constant tendency to the 

 formation of branches which traverse the wood at right angles 

 to the walls. The third phase in distribution is established when 

 the vertical strands give rise to hyphae, which are developed at 

 right angles to the original course, and which then traverse 



the tissue at right angles to the principal 

 lines of structure (plate 10). 



It might be supposed that the course 

 of the fungus would be determined by 

 the presence of physical openings in the 

 walls of the tracheids, and that the myce- 

 lium would therefore follow an irregular 

 course leading it through the various bor- 

 dered pits, which, as previously shown, 

 offer ample opportunities for such pas- 



FlG. 48. PlNUS RESINOSA. 011 



Radial section showing sa e - Such > however, is in no sense the 

 the progress of a fungus case. In fig. 48 a radial section of the red 



mycelium across the line in (p j nus reginosa) shows dearl 



of structure, and its pene- 



that the growth of the fungus is wholly 

 independent of physical openings of any 

 kind, otherwise it would take advantage of 

 those which lie in its immediate neighborhood. On the contrary, 

 its course does not deviate from the original direction established 

 at the point of emergence from the main filament. Whenever 

 in its progress the mycelium comes in contact with the cell 

 wall, its enzyme attacks the latter, and by solution establishes 

 an opening through which the fungus passes. It will be noted, 

 nevertheless, that the resistance offered by the wall is sufficient 

 to bring about a great reduction of the mycelium, which is always 

 much less than the normal diameter within the limits of the 

 wall. In thin-walled cells, where little resistance is offered, the 

 opening thus established is commonly larger than the mycelium, 



tration of the cell wall, 

 independently of the 

 presence of pits, x 350 



