664 Thoday.—On the Histological Relations between 
more nutritious cortex, where they were occasionally found applied to 
a secretory cell. The hyphae in the cortex are smaller and shorter and 
have smaller nuclei. 
Finally, in the fully developed haustorium, the brush of loose hairs has 
become a tissue of closely compacted cells, a few on the extreme periphery of 
the brush alone remaining separate from one another. It must be clearly 
realized, however, that the tissue owes its origin to the fusion of separate 
strands of cells, and that therefore all the cells composing it are not geneti¬ 
cally connected with one another. Some of the longitudinal walls have 
originally formed the walls of separate invading hyphae, and have become 
fused together later ; these walls afford an excellent opportunity for the 
investigation of the question which forms the main object of this paper, 
viz. whether connecting threads are formed between non-genetically con¬ 
nected cells. 
B. Histology. 
We are now in a position to consider in detail the manner in which the 
young invading hyphae make their way through the cortex and pericycle of 
the host, and through the old disused sieve tubes, form connexions with the 
functional sieve tubes of the host, and, finally, give rise each to a strand of 
sieve tubes which conveys the food products absorbed from the host to the 
main parasite stem. 
Fusion of lateral walls of invading hyphae to form a tissue . The tips 
of the invading hyphae are for some time separated from one another, but 
later, when the more advanced of the hyphae have been joined by those 
which lagged behind* they become closely applied to one another. Their 
adjoining walls are then fused together so closely that all sign of their separate 
origin is lost. Fig. 51 A, PI. XLIX, represents small portions of the longitu¬ 
dinal walls of two adjacent hyphae near their tips. The section was stained 
with London blue, and the blue colour in the figures indicates the distribu¬ 
tion of the bluish-green colour so characteristic of callus and substances 
like it. The inner and outer superficial layers of the walls of both hyphae 
were stained blue. Apparently the blue colour is characteristic of various 
substances produced by the hydrolysis of ordinary cellulose; 1 these, being 
mucilaginous, facilitate the fusion of the walls. The softening is a tempo¬ 
rary process and is lost when its object is accomplished. In Fig. 51 B the 
two adjacent walls have fused, and the wavy blue line marks the still visible 
softening along the line of fusion. In Fig. 51 C the line of fusion no longer 
stains blue, but can still be clearly seen (see also F, Figs. 76, 77, 78, PI. LI). 
As in Fig. 51 A, PI. XLIX, the inner layers of the walls stain blue ; in some 
cases even larger portions of the wall may be temporarily softened. Since 
the hydrolysis of the outer layer is probably affected by the ferment 
1 See Sykes, 1908, p. 315. 
