508 May brook. — On the Haustoria of 
attack was precisely similar to that on Calluna root. As has been already 
mentioned, Fig. 3, C, represents in h. 1 and h . 2 two haustoria of P edicularis 
attacking a root h.r. of the parasite. From the figure it can be seen that 
these two haustoria were similar in structure to the two haustoria, h. 3 and 
h. 4, which were attacking roots of Calluna c. 1 and c . 2. 
There can be little doubt as to the morphology of the haustorium. 
That it is not a lateral root is evident, firstly because the haustoria arise 
indiscriminately, without any regard to sequence, on the surface of the 
lateral rootlets — in one case figured (Fig. 1, c) a ha-ustorium had arisen 
actually at the point of origin of a lateral rootlet ; and secondly, because the 
seat of origin is in the hypodermal layers of the root. They must therefore 
be regarded merely as superficial outgrowths of the roots formed for the 
purpose of extracting food material from foreign roots. 
Minute Structure and Histology of the Tracheidal 
Elements of the Haustorium. 
So far no account has been given of the minute structure of the 
elements composing the conducting tissue of the haustorium, beyond the 
fact that when lignification has occurred, the cells still retain their proto- 
plasmic contents and nuclei. The penetrating cells of the piliferous layer 
differ from ordinary adult tracheides only in the possession of protoplasmic 
contents and a nucleus (Fig. 5, A, /.A). Before lignification of the walls sets 
in, the contents of these club-shaped cells appear to diminish slightly in 
density, the protoplasm lying along the walls (Fig. 5, A, No alteration 
in size of the nuclei is visible. That this slight diminution of protoplasm is 
not a stage in the complete degeneration of the protoplasmic contents, as 
would be the case in the development of an ordinary tracheidal cell, is 
demonstrated by the fact that in quite old haustoria these tracheidal 
elements are always seen to contain protoplasmic contents. 
The elements of the tracheidal connecting strand and the tracheidal plate 
differ from each other only in size, the elements of the tracheidal plate being 
larger than those of the connecting strand. They both alike differ from the 
penetrating tracheidal cells of the piliferous layer, in that their protoplasm 
exhibits a number of granules, which stain bright blue with aniline blue. 
These elements, which will be called the ‘ phloeotracheides ’, differ only from 
those of Exocarpus , described by Benson, 1 in the possession of a nucleus. 
Otherwise they possess every characteristic of phloeotracheides. As Benson 
emphatically states that the phloeotracheides of Exocarpus were devoid of 
nuclei these cells were very carefully examined. In the majority of cases 
nuclei were found, and nowhere could any sign of degeneration of the 
nucleus be perceived. It is therefore concluded that the normal condition 
1 Benson, Margaret, 1 . c., p. 671. 
