Benson.—Root Parasitism in Exocar pus. 
669 
General Morphology of the Roots and Haustoria of 
Exocarpus. 
The roots branch very irregularly, and show but an insignificant 
development of root-hairs (Text-fig. 1). They are generally of a red-brown 
colour. 
The haustoria are minute, and occur chiefly on very slender branches, 
which makes it exceedingly difficult to demonstrate them except in young 
plants which it is possible to lift with the soil still attached. Those depicted 
in Fig. A on PI. LV are but slightly reduced, but it will be found 
advisable to use a hand lens in examining them. Those drawn in Text- 
figure t are magnified about 8 diameters. The branches on which the 
haustoria occur may arise exogenously, and in one case of which I have 
a serial set of transverse sections the branching was a simple dichotomy. 
The haustoria are generally, if not always, lateral, two sometimes appearing 
near the tip of a small root. As recorded by Dr. Barber for other members 
of Santalaceae, it is generally possible to demonstrate a continuation of the 
mother root, even though, through a series of many sections, the haustorium 
appears to be terminal (cp. left-hand haustoria in Text-fig. 1). 
Anatomically the branches bearing haustoria differ from one another. 
Some contain irregularly disposed, reticulately thickened tracheides like 
those in the Thesium haustorium (Fig. 2, PI. FV, n). These I will refer to 
as ‘ necks’, and they will be found invariably to terminate in a haustorium. 
Others contain a strand of pitted tracheides with phloem, and differ only in 
the number of protoxylems from the mother root. These always spring 
from a protoxylem of the mother root, and though often adventitious are 
obviously best regarded as roots. In the case of equal dichotomy it is 
impossible to do otherwise. In a third type (that selected for illustration in 
Text-fig. 2) we see a well-developed haustorium borne on a neck which only 
differs from an ordinary root in the absence of phloem and in the presence 
of a terminal haustorium. On the whole it would appear that the roots of 
Exocarpus are (like those of Thesium) profoundly influenced in their mode 
of branching by the new functions superposed on that of absorption from 
the soil. The direction also of the growth, especially of the lateral roots, is 
affected more by the proximity of their would-be hosts than by their rela¬ 
tion to the vertical position. 
The Haustorium. 
Before proceeding to discuss the features of interest in these new 
haustoria it may be well to explain by the help of the diagram (Text-fig. 2) 
the terms applied to the various parts of the organ. This diagram repre¬ 
sents a transverse section of the Exocarpus root, E, with a haustorium 
