666 
Tho day.—On the Histological Relations between 
with again and again in which the pressure of a hypha has bulged in 
a thick-walled cell to the extent of causing its two longitudinal walls to 
touch each other, obliterating the cavity between them. Sooner or later, 
however, the solvent action of the hypha dissolves even the thick walls of 
these elements. In Fig. 52, PL L, are shown two invading hyphae which 
have penetrated for some distance into the pericycle; a portion of the 
wall of the last cell penetrated (f, Fig. 52) is still visible in the space 
between their tips. The next pericycle cells are in their turn about to 
succumb, and the wall of the cell on the right is already considerably 
bulged in, and is dissolved as far as the middle lamella. 
Occasionally when an invading hypha has made an entry into the 
cavity of a thick-walled cell it turns and for a short time pursues its course 
longitudinally in the cavity of the host cell, following the path of least 
resistance and widening the cavity by its pressure. 
The wall at the tip of an invading hypha becomes more and more 
swollen and mucilaginous as it penetrates the pericycle and reaches the 
disused phloem ; the inner layers of the wall now commonly stain with 
London blue or water blue, and are evidently softened in the same way as 
has been described in the lateral walls of adjacent hyphae which are about 
to join with one another. 1 Occasionally even in the inner layers of the peri¬ 
cycle (Fig. 53) the tip of the hyphal wall is sufficiently hydrolysed to stain 
blue; this is of some importance, since it makes it appear certain that the 
blue-staining substance is the product of some activity of the hyphal cell 
itself (probably of a hydrolysing ferment), and is not due to an interaction 
between the parasite and the old sieve tubes of the host, which contain 
much blue-staining callus. The same conclusion is suggested by the fact 
that it is the inner layers of the wall which first undergo the change. 
Fig. 55 shows a very early stage in which only two minute patches of blue- 
staining substance have yet been produced (see also Figs. 56, 58, &c.). 
Only a few cases were seen in which hydrolysis in the walls of the hyphal 
tips had gone far enough for them to stain with water blue, 2 but the less 
hydrolysed state which stains with London blue was common even in 
hyphae in the outer zones of the old host phloem. 
Sometimes the blue staining substance occurs at first only in patches 
(Fig. 61); later it generally extends throughout the inner layers. A 
thin outer layer of unstained cellulose is visible until the invading cell 
reaches the functional phloem (Figs. 56, 58, &c.). Then, on the edge of 
the functional phloem, there is a short stage in which the whole wall at the 
tip of the hypha stains blue. Being transient, this stage is not very often 
seen. Figs. 59 and 60 represent hyphae just getting to this stage; in 
Fig. 59 two small blue patches are seen in the outer layers as well as the 
large swollen mass within. Fig. 60 represents a transverse section through 
1 Ante, p. 664. 2 Sykes, 1908, p. 315. 
