672 Thoday.—On the Histological Relations between 
In the cortex the parasite hypha bores straight through the wall into 
the cavity of the cell, without apparently taking any account of the presence 
of connecting threads; in the phloem, on the other hand, it makes con¬ 
nexion with the unchanged sieve area. The junction is effected in such 
a way as to disturb as little as possible the mechanics of the sieve tube, and 
the hypha thus secures a long-continued supply of food. 1 
The adaptation of the unchanged sieve area to the purposes of the 
parasite points to the great efficiency of the multiperforate septum 2 of 
which the sieve area is composed. In the cortex of the host the connecting 
threads appear to be inadequate for bringing a supply of food to the 
hyphae. Even those hyphae which finally extend freely into the cortex 
of the host seem always to make a hole in the wall of the last cortical cell 
attacked. These facts may be regarded as affording further evidence 
in favour of the view that while connecting threads seem to be of value 
rather for the conduction of impulses (stimuli) than of food, 3 the slime 
strings of the sieve areas afford an excellent medium for the translocation of 
food substances. 
It would be of interest to know what would occur if an invading hypha 
made contact with the wall between a sieve tube and a companion cell 
of the host, i. e. with a wall crossed by connecting threads bored out only 
on the sieve-tube side as far as the middle lamella. Unfortunately no 
satisfactory cases of this kind have been observed. 4 
B. Callus. Substances which stain with callus stains are not all 
identical. I have already shown that in the seaweeds 5 there are various 
states of hydration of the cell-wall, in which it stains with one or other of 
the callus stains; in the least hydrated condition it is affected by London 
blue alone. ‘ Callus ’ is thus a collective term which applies to several 
different states of hydration, if not actually to more than one chemical 
substance. 6 
In the haustorium of Cuscuta also, there are several substances which 
stain with callus stains and which play an important part in the history of 
the parasite. There is the ordinary callus in the sieve plates which stains 
with all the callus stains and which, as has been made clear by recent work, 
is due in the young sieve plate to the hydrolysis of the cellulose wall 
by a ferment, but is added to in the later stages by accumulations of callus 
directly deposited by the protoplasm. 7 
1 It would be of interest to know what would occur if an invading hypha made contact with 
the wall between a sieve tube and a companion cell of the host, i.e. with a wall crossed by con¬ 
necting threads bored out only on the sieve-tube side as far as the middle lamella. Unfortunately no 
satisfactory cases of this kind have been observed. 
2 See Hill, 1901 b , p. 604. 
3 Hill, 1908, pp. 281-3; on the other hand, Gardiner, 1898a, p. hi (food and impulses). 
4 See p. 668, footnote 1. 6 Sykes, 1908, pp. 313-17. 
6 Cf. Mangin, 1892. 7 Hill, 1901 b, pp. 597-600, and 1908; Sykes, 1908. 
