95 
Physiology of the Genus Cuscuta. 
than before, and to the presence of the large haustorium ( H ) 
whose phloem- and xylem-elements have united with those of 
the petiole. On the upper side, the region p, which in the 
section represented in Fig. 2 was composed of ordinary 
spheroidal parenchyma-cells, has been the seat of consider- 
able changes. The cells composing it have been compressed 
laterally into elongated forms, their long axes being at right 
angles to the surface, by the bending round of the incomplete 
vascular ring which was divided and forced apart by the 
intruding haustorial wedge ( H ). These cells, elongated by 
pressure, were then divided by cross-walls at right angles with 
their long axes and parallel to the direction of the com- 
pressing force. Finally a considerable thickening and ligni- 
fication of the walls of all of these cells, and of some adjacent 
cells which had not been so much compressed and had not 
divided, took place. Thus, by forming between the two ends 
of the incomplete vascular ring a compact mass of not easily 
compressed cells, the petiole can resist, though still with little 
effect, the intrusion and growth of the haustorium. In the 
region s, as shown in Fig. 2, sclerenchyma-fibres are formed 
in small groups ; but in the section represented in Fig. 3 they 
are decidedly more numerous. The walls of these cells too 
are thicker and more strongly lignified. In other respects 
the characters and proportions of the component tissues of 
the petiole remain unaffected by the presence of the active 
haustorium. 
We have seen that peripheral tissues difficult of penetration 
(such as those of Aloe, J uncus , Equisetum) ; that vascular 
bundles so scattered and so surrounded by resistant elements 
(as in Juncus and Equisetuni) that the union of the haustoria 
with them is difficult and infrequent ; that tissues containing 
poisonous matters (as in Aloe and Euphorbia), are great pro- 
tections for a plant against the attacks of Cuscuta . Anatomical 
changes taking place after the haustoria have penetrated avail 
little, as the petiole of Solanum illustrates. But many plants 
resist attacks successfully merely because they are too large 
to be embraced by the parasite. 
