1 84 ANATOMY OF THE GYMNOSPERMS 



temperature, an absence of moisture, and, if possible, a complete 

 exclusion of oxygen. 



3. Their mode of operation. The mycelium, or plant body, 

 of the fungus consists of a very slender thread which is about 

 7 //, or less in diameter. Comparing this with the normal phys- 

 ical openings in the tissue of coniferous woods, it is found to be 

 only one fourth the size of the cavity of the tracheid (28 /x) of 

 the spring wood, while it is but little smaller than the average 

 tracheid cavity of the inner summer wood (10 /x), and twice as 

 large as the cavities of the tracheids last formed (3.5 /u) in the 

 wood of Juniperus virginiana. Within this same species it is 

 but little smaller than the tangential diameter of the ray cell 

 (8.7 n), while it is about twice as large as the average pore of 

 the bordered pit (3.5 /LA). Red cedar was selected for compari- 

 son for the reason that the structural features referred to are 

 relatively small and they represent what is common to a number 

 of species, such as those of Torreya and Taxus. But it must be 

 remembered that in the majority of coniferous woods the open- 

 ings referred to are far larger, and they would therefore offer 

 correspondingly more favorable conditions for free development 

 of the mycelium. From these facts it is not difficult to perceive 

 that when spores germinate on the surface of a timber, on a 

 wounded surface, or in a crack, the growing plant at once finds 

 ready access to the interior parts through the natural channels 

 afforded by relatively large openings in the tissue. Such entrance 

 will be greatly facilitated in proportion as the surface is rougher 

 or the tissue is in any way lacerated, since such laceration not 

 only increases the size and number of the initial openings but 

 is also a factor which contributes to more speedy disorganization 

 of the organic substance of the cell wall. From this it is evident 

 that the stumps of branches which have been left in a ragged 

 condition either through wind pruning or through the careless 

 operations of the forester must afford conditions highly favor- 

 able to the operation of decay. Or yet again, when felled timber 

 cracks in the process of drying before it is rafted, the cracks 

 offer inviting places for the lodgment of fungus spores, especially 



