220 
Transactions of the Royal Society of South Africa. 
in the cavities it has produced by the destruction of the wood. Fig. 10 
shows a collection of hyphae evidently intertwining and becoming matted 
together. 
Mycelium of the Fungus 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 /x 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 /x diam. 
and walls 6*6 /x 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 /x (on the average) thick and lumina 7'36 /x diam. The 
prosenchyma cells have simple pits. 
It is the absence of wide wood elements that makes the wood of 
Ptxroxylon 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 touchinc; the wall to be bored through. 
