220 Transactions of the Royal Society of South Africa. 



in the cavities it has produced by the destruction of the wood. Pig. 10 

 shows a collection of hyphae evidently intertwining and becoming matted 

 together. 



Mycelium op the PirNGiJS in its Host. 



In young stages the mycelium consists of delicate, thin-walled, colourless 

 hyphae, which are branched and septate. Frequently the septa are quite 

 close together. From being colourless the hyphae change through a yellow 

 ochre to a dark Vandyke-brown and become thicker walled. More frequently 

 they measure 1-1-2 /a across, though thicker and also thinner hyphae have 

 been met with. The hyphae fill the vessels, wood prosenchyma elements, 

 and medullary rays, and, as already mentioned, form a thick felted mass 

 in the cavities and in the place of the wood elements which have been 

 destroyed. 



The hyphae show the characteristic clamp connections, and usually they 

 become peculiarly intertwined and twisted, as is evident from some of the 

 illustrations. 



The manner in which a fungus penetrates and spreads through wood is 

 to some extent determined by the structure of the particular wood. It may 

 be taken that generally the course of the hyphae is through the medullary 

 rays and vessels in the first instance, and that from these points individual 

 hyphae penetrate the adjoining wood cells. 



Briefly considering the wood of Ptaeroxylon utile we note : 



(1) The medullary rays, though not easily evident to the naked eye, are 

 abundant, 1 cell broad and between 5-12 cells deep, and are composed of 

 comparatively thin-walled parenchymatous cells. 



(2) The vessels have a diameter of medium size (lumen 39-16-52-8 ^ diam. 

 and walls 6'6/u. thick), and their walls are pierced by numerous bordered 

 pits. 



(3) The groundwork of the xylem is composed of wood prosenchyma 

 cells, with walls 3'68 [i. (on the average) thick and lumina 7"36 fj, diam. The 

 prosenchyma cells have simple pits. 



, It is the absence of wide wood elements that makes the wood of 

 Ptseroxylon utile so hard and firm. 



Through wounds the fungus finds entrance into the medullary rays and 

 from them penetrates vertically and laterally into the wood elements. The 

 hyphae pass directly through the walls of the medullary ray cells (Fig. 11), 

 and the pits formed in the process are slightly enlarged. The hyphae are 

 also capable of directly boring their way through the walls of the wood 

 prosenchyma cells (Figs. 12 (a) and 13). Where the hyphae thus bore their 

 way directly through the walls a tuberous swelling was frequently noticed 

 at their ends and in proximity to or touching the wall to be bored through. 



