Physiology of the Genus Cuscuta. 61 
favourably proportioned to the effort made to secure it, and 
the Cuscuta often succumbs under such conditions. If, how- 
ever, the host be one centimetre or less in diameter, the 
seedling, unless exhausted by the length to which it has 
already been obliged to grow in order to reach a host, can 
make a number of turns, will clasp the host tightly, will form 
many haustoria in consequence of the extensive area of 
intimate contact, and will therefore be the more likely to 
obtain abundant nourishment. The maximum diameter of 
a stem or branch of a living plant around which a seedling 
can twine varies with the species of Cuscuta , the larger species, 
and naturally also the larger seedlings of the same species, 
being able effectually to embrace larger hosts than the 
smaller ones. One and a half centimetre is the mean maximum 
for C. Epilinum , and two centimetres for C. europaea and C. 
glomerata. I am unable to give the minimum. I have seen, 
however, seedlings winding successfully about stems of one 
millimetre in diameter. Since older plants, even of the larger 
and therefore, for mechanical reasons, less conformable species, 
find no difficulty in winding tightly about, and even forming 
incipient haustoria against, silk and cotton threads of a mean 
diameter of one-third of a millimetre, there is little reason for 
supposing that, other conditions being favourable, a seedling 
could not wind closely about and send haustoria into a host 
of no larger diameter. 
The part of the host attacked need not be cylindrical in 
form, as the frequent observation of seedlings, as well as older 
plants, which have applied themselves to the lamina of various 
sorts of leaves, proves. In such a case the very sharp turn 
which the stem of the parasite is able to make allows it to 
apply itself to both the upper and the under surface of the 
leaf. The whole curve thus made is an ellipse, one of whose 
diameters may be very short, the other proportionally very 
long. Such an ellipse made by a seedling around a leaf had 
a long diameter of one centimetre and a half, a short diameter 
of two millimetres. A similar curve made by an older branch 
of C. glomerata had as respective diameters a line nearly 
