Physiology of the Genus Cuscuta. 99 
things being equal, grow in the direction of least resistance. 
Manifestly there will be less resistance to its forward growth 
midway between two vascular bundles of the Maize-leaf than 
directly opposite one of them ; and against this intermediate 
region it pushes the epidermal and cortical cells which overlie 
it. It has been many times observed that even against 
strongly resisting rods of wood and glass the haustoria cause 
swellings of the stem by their growth. In the case now 
under discussion such a swelling directly over the tip of the 
haustorium is being pushed by the continued growth of the 
haustorium against the epidermal and parenchymatous tis- 
sues of the leaf which are situated between two vascular 
bundles (see Fig. 4). The epidermis of the leaf is pushed in 
by the low, blunt, conical swelling on the parasite, causing 
collapse of the larger, thinner-walled parenchyma-cells which 
are just behind it. 
Passing to a somewhat older haustorium on the same coil, 
and sectioning it and the leaf to which it is attached, one sees 
that the continued and increasing pressure finally results in 
the rupture of the epidermal cells of the leaf. The epidermis 
is an elastic, tough, strongly-resisting layer. When it is 
broken through, the further progress of the haustorium by 
means of mechanical pressure is comparatively easy until it 
meets the thick-walled and strongly lignified cells of the 
endodermis of the bundles. In every experiment which 
I performed with Zea the haustoria failed to unite with the 
bundles ; and hence the branches, which had been severed 
from the parent plants after the haustoria had penetrated the 
leaves, presently died. 
Of course in the above case the pressure brought to bear 
upon the leaf was exercised by the haustorium only indirectly ; 
it grew in size and length, and thereby pushed the adjacent 
cortical cells away from it ; these in turn pushed forward the 
epidermal cells ; these epidermal cells were rejuvenated by 
the contact and stimulated by the nutritious host to grow into 
papillae of very considerable size. In this cumulative manner 
a swelling arose on the surface of the parasite. By the con- 
H 2 
