On the Histological Relations between Cuscuta and 
its Host. 
BY 
MARY G. THODAY (SYKES), 
Girton College, Fellow of Newnhavi College , Cambridge . 
With Plates XLIX-LI. 
Introductory. 
HE questions concerning the origin of protoplasmic connecting threads 
X in plant tissues have long excited much interest. At present we are 
confronted with two main conflicting theories, namely, that of Strasburger, 1 
who has hitherto maintained that connecting threads owe their origin to 
the boring out of the cell-walls by the protoplasm (the formation of 
‘ plasmodesmic pseudopodia’), and that held by Gardiner 2 and previously 
suggested independently by Russow 2 and Kienitz-Gerloff 2 , who regard 
connecting threads as derived primarily from spindle fibres, and as therefore 
only present when cell-wall formation follows on nuclear division. It is 
obvious that if we knew whether connecting threads are ever formed 
between cells or tissues which are not genetically continuous, we should 
be a step further towards a decision between these theories. On this point 
a large amount of evidence will no doubt be necessary, but it occurred to 
me that a good beginning might be made by a careful histological investi¬ 
gation of the haustoria of parasites, in order to determine whether sieve 
plates are ever formed in compound walls separating the cells of the host 
from those of the parasite. The work of previous authors 3 on parasite 
haustoria tends to support the former of the two theories, but recent improve¬ 
ment in histological methods now makes more minute examination possible. 
Material of Lathraea squamaria and a species of Orobanche was 
preserved and examined, but both were found to be eminently unsuited 
to this kind of histological work. Various species of Cuscuta growing on 
1 Strasburger, 1901 and 1902. 
2 Gardiner, 1900 and 1907 ; Russow, 1883 ; Kienitz-Gerloff, 1891. 
3 Peirce, 1893 and 1894; Strasburger, 1901. 
[Annals of Botany, Vol. XXV. No. XCIX. July, ign.] 
X X 2 
