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The various conditions affecting the origin and the develop-
ment of the haustoria of three species of Cuscuta have been
discussed in the foregoing pages. There remain to be con-
sidered in this paper the means by which the haustoria
already formed in the parent plant make their way into the
host and attach their xylem- and phloem-elements to the
corresponding elements of its vascular bundles. In my pre-
vious paper on certain phanerogamic parasites ^ my treatment
of this part of the subject was regrettably incomplete, because,
at the time of writing, only alcohol-material was at my dis-
posal. From the living plants which it has since been my
good fortune to be able to observe and to experiment upon»
I am able to add to what I then wrote.

Manifestly in one or both of two ways only can the parasite

' Loc. cit.



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Physiology of the Genus Cuscuta. 97

ordinarily secure the penetration of its haustoria into a host ;
by mechanical pressure, or by chemical action, or by a com-
bination of both. In my previous paper I expressed the
opinion, based on the appearance of the excellent alcohol-
material which I had examined, that the haustoria made their
way by solution. I still believe this to be true, and I shall
presently give some experimental evidence in favour of this
view. I said nothing about pressure, for the evidences of
pressure were very slight. The following experiments
demonstrating that both solution and pressure accompany
the intrusion of the haustoria into the host will show how
necessary it was to supplement purely histological examina-
tion by physiological experiment. The plants used in the
experiments now to be described were either C. glome'
rata or C europaea, the plants of C. Epilinum by the time
these experiments were begun having almost ceased to
vegetate, developing only flowers and fruits.

If a branch of C glomerata of suitable age be brought into
contact with such a plant as Zea Mais at a part small
enough in diameter to allow the branch to make close coils
about it, haustoria will be produced. The branch should not
be placed against the stem, which is protected by a peripheral
ring of hard sclerenchyma, but rather against the base of
a leaf still wrapped around the young stem, the whole
diameter of stem and enclosing leaf being not more than one
centimetre. After the haustoria first formed have penetrated
the leaf, the leaf and the attached parasite should be cut
away. Selecting, by the aid of a hand-lens, a haustorium
which has developed to the point of entering the leaf but has
not yet done so, careful transverse sections of the leaf should
be made at this point, passing through and including the
parasite which is attached to, though it has not yet penetrated
into, the host. (The method of attachment is discussed in
division II. 2 of this paper.) Taking for examination the section
through the centre of the haustorium, the structure of the leaf
will be seen to be as follows (see PI. VIII, Fig. 4). The
upper surface (the lower, c-d, in the figure), which so near the

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98 Pdrce. — A Contribution to the

base is of course the inner one and is more or less closely
applied to the stem, consists of rather small' thin- walled
epidermal cells whose continuity is rarely broken by stomata.
Immediately underlying this layer are the usual spongy
parenchyma-cells which make up the larger part of the
thickness of the leaf. At fairly regular intervals groups of
these cells, about ten in each group, immediately underlying
the epidermis, become more thick-walled and elongated,
forming slender strands running lengthwise in the leaf, and
thus adding something to its strength. The parenchyma-cdls
in the centre are large, thin-walled, spheroidal, and are
adjoined by parenchyma-cells of similar character except that
they are smaller in proportion as they are nearer the surfaces
of the leaf. The under part of the leaf, the outer owing to
its vertical position in embracing the stem, contains the large
vascular bundles, whose course is longitudinal. These bundles
project into the mesophyll, are separated from one another
by considerable masses of parenchyma, and are surrounded
individually by strongly lignified sheaths. The endodermis
of each bundle is continuous with a plano-convex mass of
sclerenchy ma- cells which abuts by its plane side upon the
epidermis. The epidermis of the lower side of the leaf con-
sists of smaller cells with thicker walls, and is more frequently
broken by stomata, than the epidermis of the other side.
Except directly opposite the vascular bundles, where it
adjoins the masses of sclerenchyma-cells, this lower epider-
mis, like the other, is in contact with the mesophyll-
parenchyma.

Turning now to the parasite, we notice that the papillate
cells of the epidermal cushion, which overlies the developing
haustorium, have become so firmly applied to the epidermis
of the leaf that their tips are flattened. The diameter of the
whole cushion is about twice as long as the distance between
two vascular bundles in the leaf. The young haustorium,
developing in the cortical matrix of its parent, will be
variously influenced by the resistance offered to the pressure
which by its growth it exerts, and it will naturally, other



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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
r^on 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 agamst 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 g^ow into
papillae of very considerable size. In this cumulative manner
a swelling arose on the surface of the parasite. By the con-

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lOO Peirce, — A Contribution to the

tinuation of these processes the swelling constantly increased
in size, but more especially in height, and pressed the epider-
mal cells which had already been for some time in contact
with the host more and more strongly against it. From
their nature and arrangement the peripheral tissues of this
host could not resist or transfer the pressure; they were
forced to bend in and collapse ; and so the pressure became
evident. A moment's study of the much more delicate and
yielding stem of Impatiens Balsamina will show how its peri-
pheral tissues are affected by the pressure of the growing
haustorium. Immediately underlying the single-layered epi-
dermis which covers this essentially cylindrical stem, are five
layers of collenchyma-cells which form a fairly strong and
decidedly elastic ring about the deeper tissues. Within this
ring are five or six layers of large rather thin-walled paren-
chyma-cells which in turn enclose the more or less continuous
ring of vascular bundles. There is an abundant central pith.
We see that the stem is composed of four concentric rings
enclosing a core of pith. These rings are not rigid, for they
are composed, except the wood, of elastic and not thick-
walled cells with larger or smaller air-spaces between them.
Even the wood is composed of comparatively thin-walled
ducts and fibres, and is so frequently traversed by medullary
rays that this ring also is anything but rigid. Hence pressure
exerted upon one part of the stem can be transferred to and
divided among all parts, a slight change of form of the whole
stem allowing the cells in the region first pressed upon to
escape for a considerable time much if not all injury from
this cause. Furthermore, the structure of the stem is such
that horizontal pressure is not only distributed among the
cells on the same plane, but among those above and below
also, and so its effects are rendered still less evident. If it
were not that the host is tightly embraced within the close
spiral formed by the parasite, we could conceive of its being
able, by sufficient change of form, to escape being penetrated
by the haustorium which, after all, is able to exert only a
limited amount of pressure ; but no stem strong enough to be



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Physiology of the Genus Cuscuia. loi

erect is able to do this, especially when it is enclosed by the
parasite wound tightly and many times about it, for it is not
yielding enough.

That the pressure exercised by the growing haustorium of
C. glomerata is very considerable, though so completely
masked by the yielding nature of the stems and petioles of
the species of Impatiens upon which it is parasitic, may be
readily demonstrated thus. Wind a sheet of tin-foil four or
five centimetres long and only wide enough to meet, not to
overlap, around a stem or erect branch of Impatiens of suitable
size, fastening it by binding bast. Bring into contact with the
stem thus enwrapped a healthy branch of the parasite. Close
coils will be formed and haustoria will also be induced by the
contact. Their formation and growth cause the usual swel-
lings, and these press with constantly augmenting force
against the tin-foil. Finally the foil is ruptured, tearing in
irregular fashion, and thus allows (see Fig. 5) the epidermal
cells covering the irregular protruding mass to come into
contact with the host. The haustorium continues to grow
and presently makes its way into the host. If the tin-foil be
wound twice about the host, the haustoria fail to penetrate
the foil, though making marked impressions in it ; and they
become abortive just as when they are formed in consequence
of contact with other innutritious supports such as glass or
wood. The tin-foil used in this experiment was two-tenths
of a millimetre in thickness and of good quality. Through it
the host could exercise no chemical influence on the parasite,
nor could the parasite excrete a solvent of the metal. Hence
wc see that very considerable pressure is exerted by the
growing haustorium and the epidermal cells immediately
overlying it, and that pressure alone is sufficient to accomplish
penetration into the host.

Such pressure is paralleled by that exercised by normal
roots, whether they be lateral or main ones. Prunet^ has
again called attention to this in a paper on the penetra-

* Pranet, M. A., Sur U perforation des tabercnles de Pomme de Terre par les
rhizomes des Chiendents, Revue G^^rale de Botanique, III, 1891.



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I02 Petrce. — A Contribution to the

tion of potato-tubers by the rhizomes of * Quitch Grass ^
{Agropyrum repens^ Beauv.). From experiments of my own
on seedlings of various plants producing, both large and
small main roots, it is perfectly evident that the power of
penetrating living tissues by pressure alone is not a peculiarity
of haustoria (which are but lateral roots modified in origin and
structure to accomplish a very special purpose), nor of the
roots of some Grasses. If a root be so firmly fixed against
a plant that, if it grows at all, it can only grow into the
opposing tissues, it will invariably penetrate them. Into how
solid tissues these ordinary roots can make their way it is not
within the scope of this paper to discuss. It is sufficient now
to state that they readily penetrate through tissues as solid as
the cortical tissues of any of the hosts of Cuscuta which I have
yet seen. As recently determined by Pfeffer^, the pressures
which ordinary roots are able to exert are great. Accurate
determinations of the pressures exerted by haustoria are, on
account of the habits of the plants, most difficult to make ;
but from the celerity and apparent ease with which the
haustoria of C. glomerata penetrate tin-foil it is hard to
suppose that they are the weakest of all roots.

From the foregoing experiment it becomes evident that,
though one object of the close coils formed about its host
is to produce intimate contact of a considerable area of the
parasite with its host, in order to induce the formation of
numerous haustoria, there is still another and also a very
important object. Owing to the closeness of the coils and
to the large area consequently applied to the host, any force
acting between the parasite and the host and tending to push
them apart, would be resisted by the very great friction which
must be overcome before the parasite could be made to uncoil.
This resistance is not far from the breaking-strength of the
stem. The force developed by the growing haustoria tends
to push host and parasite apart, and it is resisted in this way.
But the matter is not so simple. After a branch has made

* Pfeffer, W., Druck- nnd Arbeitsleistnng durch wachsende Pflanzen. Abhandl.
d. K. S. Gesellschaft d. Wissenschaften, XXXIII, 1893.



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Physiology of the Genus Cuscuta. 103

a close, nearly horizontal turn about the host, the curved part
still continues for a time to grow in length. It is true that
this growth is neither rapid nor of long duration, yet it is
sufficient to loosen the spiral somewhat, to weaken the con-
tact, to reduce the stimulus for haustorial formation, to lessen
the force which the growing haustoria can bring to bear on
the host, by increasing the distance through which they must
grow. However, accompanying and following this compara-
tively slight growth in length is a very considerable and long-
continued growth in thickness. The result of this latter is
more than to counterbalance the gjrowth in length. It main-
tains the closeness of the spiral, it constantly increases the
intimacy of the contact, it strengthens thereby the stimulus
for haustorial formation, and supplements the force which the
growing haustoria can bring to bear on the host, by reducing
the distance through which they must grow. This growth in
thickness further supplements the force by (first) causing the
stem of the parasite to press against and thus to compress
the host by its whole concave side, and not merely by the
swellings over the young haustoria ; and (second) by increas-
ing the rigidity of the coils so that, firmly braced by the
coiled stem in which their strong bases are embedded, the
haustoria can apply all their force forwards and against the
host. Besides, the parasite is not merely closely pressed
and firmly held against the host, but, as I shall demonstrate,
is actually attached to it by many of the epidermal cells
covering the swellings over the haustoria.

That the stem of the parasite, whether it bear haustoria or
not, and whether it be coiled closely or only steeply and
loosely around the host, exercises a very considerable pressure,
might be suspected from its resemblance to tendrils and
twining stems, but it may be made quite evident by the
simple method used in demonstrating, though not thereby
measuring, the pressure of tendrils, and the relative pressures
developed by the two sorts of coils will at the same time be
indicated. Let a branch of Cuscuta which is irritable, and
therefore in condition to make close turns, be brought into



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I04 Peirce. — A Contribution to the

contact with a glass rod whose average diameter is known;
and another branch, which is not in the irritable stage, be
brought into such a relation with another rod of the same
diameter that it will make steep turns about it. After both
branches have twined about the supports for from thirty-six
to forty-eight hours, remove the rods with as little disturbance
of the coils as possible. It will at once be noticed that the
diameter of both coils decreases, and that the number of turns
increases proportionally. After waiting from twelve to
twenty-four hours, the contraction will have reached its
maximum and have become permanent. By measuring now
the average diameters of the two imaginary cylinders enclosed
by the gradual and the steep spirals respectively, by the close
turns formed in consequence of irritation and by the loose
turns formed without irritation (that is, as a climbing plant
would form them), we find that the diameter of the former is
from one-half to three-quarters the diameter of the rod first
employed, whereas the diameter of the latter is never less than
three-quarters the diameter of the rod. Evidently the steep
spiral, formed only for supporting the plant as it ascends,
exercises little if any more than enough pressure on the
support to keep the plant from slipping down by its own
weight; whilst the close spiral, formed in consequence of
irritation for the purpose of forming an intimate and enduring
contact and for enabling the haustoria the more readily to
penetrate the host, exercises very considerable pressure. As
shown before, the pressure exercised by the closely coiled
stem is still more increased by the formation and growth of
the haustoria.

But the force brought to bear on the host by the parasite,
whether through the gradual or the steep spirals, is not merely
one of compression; for by its circumnutation the parasite
exercises a force which, unfortunately, has not yet been
determined. We must therefore recognize two sorts of force
which are applied to the host ; a deflective, exercised by
circumnutation, and a compressive, exercised by the coils and
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Physiology of the Genus Cuscuta. 105

of these two, when it is possible to estimate them in
mechanical units, will be found surprisingly large when
the delicacy of the plant developing them is taken into
consideration.

%. Penetration by the chemical activity of the pre-haustorium.

Von Mohl ^ noticed that when a Cuscuta had wound about
a polished silver rod, the positions of the haustoria against
this rod were indicated, after the plant had been untwined
from the support, by spots of a glairy fluid or of a shiny dry
deposit. This he believed to be a mucilaginous matter which
the parasite exuded in order the better to fasten itself to the
host L. Koch ^ considers it to be a solvent which is secreted
for the purpose of softening if not dissolving the opposing
tissues of the host, but he gives no experimental proof of this
hypothesis. There are in the host three sorts of matter which
may be affected by the parasite : the cell-walls, composed of
cellulose and more or less inflltrated matter ; the starch and
other carbohydrates in solid form ; and the nitrogenous sub-
stances. Though the haustorium may be able by mechanical
pressure alone to penetrate the tissues and to bring its
vascular elements into direct contact with the phloem and
xylem of the host, yet it goes without saying that it would be
greatly to the advantage of the parasite were it able to
supplement physical force by chemical action, and thus not
only to make penetration easier, but also to allow the cells of
the haustorium to dissolve, and thus to bring into available
form, the solid nutritive matters around them. I have demon-
strated in the foregoing pages that both contact and nourish-
ment are necessary to the full development of haustoria.
I shall now describe certain experiments showing some of
the ways in which this nourishment is obtained.

Let a mixture of two parts plaster of Paris and one part
of starch (I used the large-grained starches of potato and
barley), very thoroughly stirred together, be wetted with

* Mohl, H. v., loc. cit (cited by Koch on p. 55).

* Koch, L., loc dt, 1880, pp. 56, 57.



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io6 Peirce. — A Contribution to the

a small quantity of water and cast into rods of about six
centimetres length and less than one centimetre diameter.
These may be sterilized just before using by soaking for
fifteen minutes in ether and then rapidly evaporating under
diminished atmospheric pressure. Into a moist chamber
made of large glass-tubing, such as I have previously described
(page 75), which has been thoroughly sterilized, insert the
irritable tip of a branch of C. glomerata^ and by sterilized
forceps place the rod of plaster and starch in the chamber and
in contact with the branch, quickly closing the chamber. In
the course of six days the branch should have twined and
developed several haustoria. Cutting the branch from its
parent, remove it, still in contact with the rod, from the
chamber. By a clean flat-pointed needle remove some of the
starch and plaster from directly under a haustorium and
examine it in water under a microscope. With another clean
flat needle remove some of the starch and plaster from some
part of the periphery of the rod far removed from any
haustoria, and examine it also in water. It will be noticed
that on the first slide, of material from under a haustorium,
the proportion of starch to plaster is smaller than in the
original mixture, and that most of the starch-grains still to be
found are corroded in various d^^ees (see PI. VIII, Fig. 6).
The corrosion proceeds in the majority from the centre of the
grain toward the periphery, forming broad and more or less
radial canals ; but a very considerable number of grains show
the corrosion beginning at the periphery and running in, or
beginning at several points between periphery and centre.
In the last case, the corrosion proceeds from several centres
instead of from one, but finally a common cavity is made by
their union in the centre of the grain. Examination of the
second slide, of material far from the influence of haustoria,
will show that the proportion of starch to plaster is approxi-
mately the same as that of the original mixture ^, and that
none of the starch-grains are corroded.

^ It is scarcely to be expected that the ingredients should have been so perfectly
mixed that their proportions would not vary somewhat in different parts of the rod.



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Physiology of the Genus Cuscuta. 107

If now the tip of a haustorial swelling be cut off and
examined with a little of the starch and plaster still adhering,
it will be seen that the haustorium has not penetrated the
overlying epidermis, but that the epidermal cells have become
papillate, and that most of the starch-grains still existing,
either in contact with or very near the papillae, are deeply
corroded. The corroded starch-grains can more accurately
be observed, however, if after cutting off a bit of the stem
in contact with the rod, some of the mixture still adhering
to the tips of the haustorial swellings be removed to a slide,
spread out thereon, and examined in a drop of water.

Evidently then the starch in contact with and in the
vicinity of the papillate epidermal cells has been acted upon
by them. That starch-grains not in contact with these cells
are also acted upon shows that the ferment which is secreted is
capable of diffusion for some distance through plaster of Paris,
however limited may be its power of diffusion through
colloids ^. Examination of the plaster and starch with which
unmodified epidermal cells are in contact, that is, between the
haustorial swellings, shows no corrosion ; the starch-grains



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