Benson.—Root Parasitism in Exocarpus. 675 
Rhinanthaceae. 1 The phloeotracheides form a head and an axile strand, 
while the translucent cells which are axile in Santalaceous haustoria sur¬ 
round the axile strand. Thus all the same types of cell are present, but are 
distributed differently. 
Fig. 2, PI. LV, exhibits a section through a Thesium root attacked 
by another Thesium root. The haustorium is borne laterally on a small 
diarch root and shows the apparently interrupted continuity between the 
haustorium and the mother root. It is noticeable that the surface of con¬ 
tact between parasite and host is scarcely distinguishable. For this cause 
the cells of the cortex are not entered in the drawing. Fig. 2 a is an enlarged 
drawing of part of the cells abutting on one another, and no trace of 
disintegrated cell-wall is visible. This case is in harmony with an observa¬ 
tion made by Dr. Barber of a comparable case in Santalum. Dr. Barber 2 
noted that when Santalum haustoria had become fixed to the roots of 
Santalum actual fusion of the tissues seemed to occur, and pointed out that 
it would appear to be more or less natural that such fusions should occur. 
Such cases suggest that when a Thesium root attacks one of its own 
kind a state of equilibrium is reached, and possibly both roots share the 
supplies. Opposed, however, to this view is the fact that only a trace 
of starch is to be seen in the medullary rays of the host root, although 
in other cases the Thesium root is charged with starch. Moreover, Barber 
describes an interesting case in which a Santalum root, when undergoing 
self-attack, had proceeded to occlude the region affected and make some 
effort to throttle the sucker. 
Discussion as to the Function of the Haustoria. 
In one series of sections the sucker of a Thesium haustorium has pene¬ 
trated and split into two halves the stele of a grass root, and is shown in the 
act of absorbing the thickening layers on the wall of the further part of the 
endodermis. This and many other sections suggestive of the same thing 
show us that the haustoria of these hemiparasites do not limit themselves to 
an attack on the water-carrying elements. I mention this because it seems 
rather overlooked in the literature. Thus Haberlandt 3 says of the absorb¬ 
ing cells: ‘ Sie setzen sich auf kiirzestem Wege mit den Gefassen der 
Nahrwurzel in Verbindung.’ 
Solms-Laubach 4 also, while figuring only such cases of root parasitism 
as do not penetrate beyond the wood, says :—‘ Der Saugfortsatz ist genothigt, 
die sehr feste Schutzscheide und das Holz zu spalten, um seine Gefasse 
denen der Graswurzel anlegen zu konnen.’ 
Such expressions, though true, are liable to be misunderstood, for there 
1 Cf. Solms-Laubach, 1 . c., Taf. XXXIV ; also Leclerc du Sablon, 1 . c. 
2 Barber, l.c., Santaluni album , Pt. II, p. 44. 
3 Haberlandt, Pflanzenanatomie, 3. Auflage, 1904, p. 225. 
4 Solms-Laubach, l.c., p. 549 ; also Tafel XXXII, Fig. 4. 
