Anton Kerner von Marilaun.

The natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) online

. (page 63 of 128)
Online LibraryAnton Kerner von MarilaunThe natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) → online text (page 63 of 128)
Font size
QR-code for this ebook


egg-shaped bladders of golden or reddish yellow colour, and each contains the
parasite, or rather its spores. The parts of the leaf attacked by Synchytrium
Myosotidis are also much thickened, the palisade cells and the air-containing
lacunae of the spongy parenchyma (cf. vol. i. p. 279) disappear, and the tissue
consists entirely of large similarly-shaped cells which fit close to one another,
leaving no spaces between. In the gall caused by Synchytrium pilificum on
Potentilla Tormentilla the much-enlarged cells in which the parasite settles are
overgrown by the adjoining hypertrophied cells, some of which rise up in the form
of hairs, and the whole new structure resembles a hairy wart.

A curious gall is produced by Exobasidium Rhododendri on a sharply-defined
portion of the foliage-leaves of the Alpine Rose (Rhododendron hirsutum and
ferrugineum). A spherical spongy body rises from a restricted portion of the
leaf, usually from the under side of the somewhat projecting midrib, sometimes
only as large as a pea, sometimes as big as a cherry, and occasionally even attaining
the dimensions of a small apple. It is yellow, but rosy-cheeked like an apple on
the side turned to the sunlight, and it reminds one of this fruit by its succulent
tissue and sweet taste. Indeed, these galls are sometimes called "Alpine Rose-
apples ". Their surface is covered with a bloom which is caused by the numerous
spores developed there and does not consist of wax like the bloom on an apple
rind. The neck joining the gall to the leaf is not more than 1-2 mm. across, and,
what is still more remarkable, except for this sharply-defined place of connection
the infected leaf is unaltered.

Galls produced by the Gymnosporangia on the leaves of the Mountain Ash,
Pear-tree, Rock-medlar, and other Pomese exhibit strange forms. One of them,
caused by Gymnosporangium conicum, on the foliage of the Rock-medlar (Aronia
rotundifolia), is represented in fig. 357 2 . It resembles a tubercle furnished with
horns projecting from the lower surface of the leaf. Microscopic examination
shows that the knob consists of the strangely metamorphosed spongy parenchyma
of the leaf. The intercellular spaces which normally contain air are quite filled
with the mycelial threads, and in the projecting portion of the tubercle, which
is very hard and almost cartilaginous, tubes are inserted which terminate blindly
below, where the spores of the parasite are developed, whilst above they are open
and fringed, thus allowing the spores to escape. These tubes look like horns to
the naked eye. Usually several galls occur together on the same leaf. They are
conspicuous at some distance on account of their colour. The chlorophyll is
destroyed wherever the mycelium of the parasite extends and a reddish-yellow



FUNGUS-GALLS ON STEMS.



521



colour takes its place, so that orange spots appear on the surface of the foliage,
contrasting vividly with the green of the unaltered portions of the leaf.

Galls rising from sharply denned parts of the stem are comparatively rare.
One of the most remarkable is produced on the stems of a Laurel (Laurus
Canariensis) by the parasitic Exobasidium Lauri. When it appears above the
bark it looks like an aerial root, but rapidly grows into a branched spongy body
8-12 cm. long similar in appearance to one of the Fungi belonging to the family
Clavarieae (cf. fig. 195 1 , p. 21). The galls produced by Entyloma Aschersonii and
Magnusii on the Composites Helichrysum arenarium and Gnaphalium luteo-album




Fig. 357. Fungus-galls.

i Gall on the stem of the Juniper (Juniperus communis) produced by Gymnosporangium clavariatforme. s Gall on the leaves
of Aronia rotundifolia produced by Gymnosporangium conicum.

take the form of outgrowths, varying from the size of a pea to that of a walnut,
developed from special spots on the root. Whether the spherical tubercles growing
on the root-fibres of many Leguminosae, especially those of the Bird's-foot Trefoil
(Lotus corniculatus), the Fenugreek (Trigonella fcenum-grcecum), Lady's-Fingers
(Anthyllis Vulneraria), Lupin (Lupinus variabilis), and the Liquorice (Qlycyrrhiza
glabra) are to be regarded as true galls caused by the Bacteria -like organisms
invariably to be found in their interior is questionable. According to the most
recent investigations they are the outward expression of a case of symbiosis and
not of pure parasitism.

Gall developments which involve whole roots or rootlets are found on the Alder
(Alnus glutinosa), and on the Cabbage (Bmssica oleracea). The gall which is
produced on Alder roots by Frankia Alni attains the size of a walnut and has a



522 ALTERATION OF FORM BY PARASITIC FUNGI.

curious gnarled appearance; all the fibres of the root-branch thicken in a club-like
or tuberous manner and become twisted and entangled with one another. The so-
called "Fingers and Toes", caused by the Myxomycete (Plasmodiophora Brassicce),
is a gall-like hypertrophy on the root of Brassica oleracea, which not uncommonly
grows to the size of a man's head.

Many woody plants have galls which alter the internal structure as well as the
outward appearance of large tracts of the stem. The parasites settle in the corti-
cal parenchyma, producing hypertrophy there, and afterwards the most varied
distortions and alterations in the wood of that region of the stem. The trunk,
branch, or twig becomes much swollen or knotted and the cortex rent and torn.
Resin or a gummy mucilage sometimes runs out of the rifts in the gall. As such a
parasite exercises its metamorphosing faculty for several years, the canker (as it
may be termed) increases in size continually. Sporangia of varied form and colour
appear annually on the affected places, and again disappear when they have shed
their spores. The part of the stem or branch above the cankerous cushion dwindles
and dies off sooner or later. It rarely happens that the tree or shrub is able to rid
itself of the parasite. Occasionally a growth of wood and cork from the adjoining
healthy part walls in the cankerous spot so that the parasite is destroyed. The
gall produced by Gymnosporangium clavariceforme on the trunks and branches
of the common Juniper (Juniperus communis) is an example of this form (see
fig. 357 x ). From the hypertrophy there project in the early spring golden-
yellow tongues (shown in the figure) consisting of masses of spores embedded in
mucilage. Other similar growths are produced on species of Juniper by Gymno-
sporangium conicum, G. Sabince, and G. tremelloides, but it would take too
long to describe their differences in detail. It is important to mention, however,
that each of these parasites has two stages of development, living on different
hosts, the hypertrophies as well as the associated spore-producing organs of the
parasite being different in the two cases. The "^Ecidium stage" produces carti-
laginous swellings (see p. 520) in definite spots on the foliage of various Pomeae
(Aronia, Cratcegus, Pyrus, Sorbus), the " Teleutospore stage" thickenings and
tuberous outgrowths on the trunks of Junipers (Juniperus communis, excelsa,
Sabind), and these parasites can travel from one host to the other in turn. (The
two stages on different hosts are shown in fig. 357; these are not of the same fungus,
but of nearly allied ones, and illustrate the point mentioned.)

The parasite Peziza Willkommii attacks the trunks and branches of the Larch
(Larix Europcea), and produces the well-known Larch-disease or "Larch-canker".
The parasite having gained access at some point on the stem or branch first pene-
trates the cortical parenchyma, and affects the cambium so as to prevent the
further development of wood in that place. The development of the wood on the
opposite side of the stem, i.e. the formation of annual rings, may proceed for
several years, and in this way the attacked spot on the trunk takes the form of a
depression, which is rendered the more conspicuous should the wood and cortex
surrounding the parasite have undergone a greater thickening than usual. In



ENTIRE LEAVES AFFECTED BY FUNGI.



523



time the patch becomes a sunken, blistered hole from which resin flows; and every
year the fructifications appear above the cortex in the form of numerous little
cup-like structures which are white outside and scarlet-red in the concavity. As
the disease progresses the infected patch gradually spreads, and infected trunks and
branches can be easily distinguished at a distance. Towards the end of summer
the needles on the twigs above the canker turn yellow, while those on the healthy
branches are still a beautiful green. This premature discoloration is a sure sign
of the speedy death of the whole bough. A similar canker is produced on the




Fig. 358.-Various Galls.

1 Gall on the bract-scales of the pistillate flowers of the Gray Alder (Alnus incana) produced by Exoascus Alni-incance.
2 Inflorescence of Valerianella carinata. * The same inflorescence with galls produced by a gall-mite. Leaf rosette of
the House-leek (Semperoivum hirtum). Leaf rosette of the same plant which has been attacked by the fungus Endo-
phyllum Sempervivi and has become hypertrophied.

Silver Fir (Abies pectinata) by ^Ecidium elatinum, but instead of being only on
one side of the branch, as in the Larch, it forms a uniform swelling all round it.
Cankers of this kind are produced by a Bacterial organism (Bacillus amylovorus)
on fruit-trees (Apple, Pear, &c.), and on various trees belonging to the Amentiferae
(Beeches, Hornbeams, Oaks, &c.) by the Fungus Nectria ditissima.

When whole leaves undergo hypertrophy of the kind we have particularly
remarkable changes of form. For example, the normal leaves forming the rosettes
of the House-leek (Sempervivum hirtum; see fig. 358 4 ) are broadly obovate in
form, being little more than twice as long as they are broad. The leaves of the
same plant after they have been attacked by the parasitic Endophyllum Semper-



524 ALTERATION OF FORM BY PARASITIC FUNGI.

vivi (see fig. 358 5 ) are seven times as long as broad and linear in shape. They
stand erect, and are of a much paler colour than the healthy leaves. The Wood
Anemone (Anemone nemorosa) affords another example (see fig. 259, p. 229). It
spreads by creeping stems under the surface of the ground, and forms small
colonies in light thickets and in meadows. The plants consist partly of flowering
lateral shoots, and partly of foliage-leaves, which emerge above the ground from
the creeping underground stem. In normal leaves the erect petioles are all the
same length, and the leaflets are extended at about the same level. But when the
JEcidium stage of Puccinia fusca has settled on them this becomes altered. The
blades of the infected leaves tower over their healthy neighbours in consequence of
the elongation of their petioles, whilst their leaflets are smaller and less divided.
The length of the petiole in normal leaves is some 12-13 cm., in hypertrophied
leaves 15-18 cm.; but the size of the altered segments, compared with those of
normal leaves, is as 5:7. Similar changes are observed in leaves of Soldanella
alpina when attacked by Puccinia Soldanellce. The petioles of the infected
leaves are 2-4 times as long as the normal ones, the blade is smaller and
hollowed like a spoon instead of being flat, and the colour is an ochreous yellow
instead of a dark green. The same alterations in the length of the petiole,
and in the size and colouring of the leaf-lamina, are produced in the leaves of
Alchemilla vulgaris by Uromyces Alchemillce and in those of Phytewma orbi-
culare by Uromyces Phyteumatum. To this class belongs also the so-called
" curl " disease of Peach and Almond trees, produced by Exoascus deformans, and
rendered conspicuous by the considerable enlargement, undulation, and bladder-
like expansion of the infected leaf -surface, which acquires generally a very brilliant
coloration.

Floral-leaves are comparatively seldom metamorphosed by Fungal parasites.
In the Alder (Alnus glutinosa and incana) the bracts of the pistillate flowers are
changed by Exoascus Alni-incance (=E. amentorum) into elongated purple-red
spatulate lobes much twisted and bent (see fig. 358 l ) ; Peronospora violacea some-
times causes the stamens to change into petal-like structures in the flowers of Knautia
arvensis, so that they then seem to be " double "; Ustilago Maydis causes a growth
of tissue in the pistillate flowers of the Maize, the result being that instead of grains
irregular cushion-like structures 7 cm. in diameter are produced. Taphrina aurea,
which settles on the pistillate flowers of Poplar (Populus alba and tremula) causes
the ovaries to form golden-yellow capsules more than twice the usual size. The
galls produced by Exoascus Pruni on the ovaries of wild Plum, Bullace, Sloe, and
Bird Cherry (Prunus domestica, insititia, spinosa, Padus) belong also to this class.
The tissue of the ovary increases in size, but not in the same way as in fruit forma-
tion. The resulting body is flattened on two sides, brittle and yellow; the seed
inside is abortive, and a hollow space is left in its stead. The gall produced from
the ovary of Prunus domestica has the form of a rather curved pocket, which
looks as if it had been powdered outside with flour at the time the spores ripen.
These hypertrophies, which are popularly termed "pocket-plums", "bladder-plums",



ENTIRE SHOOTS AFFECTED BY FUNGI. 525

&c., fall off the trees at the end of May. They are eaten in many districts, but
have an insipid, sweetish taste.

Galls consisting of whole shoots, both the stem and its leaves being altered by the
parasite, are found principally on trees and shrubs, and only rarely on herbaceous
plants. Examples of the latter, however, are furnished by the metamorphosed
shoots of the Shepherd's Purse (Capsella Bursa-pastoris) produced by Cystopus
candidus and Peronospora parasitica. Here the leaves, especially the floral-leaves,
as well as the ground-tissue of the stem undergo pronounced hypertrophy. The
petals, which measure only 2 mm. in length in a healthy plant, may become even
15 mm. long; the sepals also elongate, become fleshy and brittle, and are distorted
and crumpled in all manner of ways. Only six stamens are developed in normal
flowers, but in hypertrophied specimens there are often eight. The metamorphosis
produced by Uromyces Pisi in one of the Spurges, Euphorbia Cyparissias, is even
more remarkable. The stem elongates far beyond its usual dimensions, and the
leaves, which are crowded together on normal shoots, are thus separated by con-
siderable intervals. The distance between two adjoining successive leaves in the
healthy Euphorbia Cyparissias is only 0*5 mm., but in the hypertrophied specimens
it becomes 2-3 mm. Infected shoots on an average are twice as high as healthy
ones. The foliage-leaves, which are thin, flexible, linear, and twelve times as long
as they are broad in the healthy plant, become, in the infected specimens, thick,
brittle, elliptical, and only 2-3 times as long as they are broad. The bluish-green
colour of the normal plant is changed into a yellow-ochre tint, and this contributes
not a little to the odd appearance of the plant. Affected plants are not uncommon
in Switzerland; a locality in which this disease has been very prevalent in recent
years being Saas-Fee in the Saas-thal. The metamorphoses produced on the shoots
of Periwinkles (Vinca herbacea, major, and minor) by the Uredospore-stage of
Puccinia Vincce and on shoots of Cirsium arvense by the Teleutospore-stage of
Puccinia suaveolens are very like those of the Euphorbia just mentioned, since
the stem becomes much elongated and the leaves shorter, broader, yellow, and brittle.
When flowers are developed on these affected shoots, they are more or less abortive
and sickly, and no fruits or fertile seeds arise therefrom. Frequently the shoots
blossom prematurely. For example, we can at once detect by its elongated rosette-
leaves when Primula Clusiana and minima are infected by Uromyces Primulce
integrifolice, and it may be observed when this is the case that the shoots do not
wait until the next spring to develop the flowers laid down in the summer, as usual,
but open them in the autumn of the same year instead.

The Cowberry (Vaccinium Vitis-Idcea) is especially worthy of notice among
low woody plants, because two kinds of parasite attack its shoots. Melampsora
Gceppertiana, in the Teleutospore-stage, causes a marked, gouty thickening in the
cortical parenchyma, which is converted into a spongy tissue; at first it is flesh-
coloured, but soon assumes a chestnut-brown tint. The stems elongate very much
and grow vertically upwards; and when several of them close together are thus
attacked they present a besom -like appearance. The foliage-leaves are much



526



ALTERATION OF FORM BY PARASITIC FUNGI.



farther apart than in the healthy plant on account of this stretching of the stem.
The lower leaves of the shoot are transformed into small fringed scales, and the
upper ones are so much shortened that their outline becomes almost circular. The
second parasite to which the Cowberry shoot is subject is Exobasidium Vaccinii (a
near ally of the already mentioned Exobasidium Lauri, p. 521). The stem becomes
pale rose-red colour, and rather thickened and spongy, but it does not elongate
much more than usual; the leaves become blistered and curiously convex on the




Fig. 359. A Witches' Broom on the Silver Fir, produced by jEcidium elatinum.

under surface. The substance of the infected leaves becomes brittle and loses its
chlorophyll. A red tint appears in place of the green, especially on the upper
surface of the leaf, whilst the lower surface, on which the spores develop, looks as if
it had been dusted over with flour. Usually the buds develop prematurely on these
shoots, i.e. the buds which, under ordinary circumstances, would not develop until
the next year push out and form new shoots shortly after they have been laid down.
The axes of these shoots, however, remain short; their leaves are closely crowded,
red in colour, and sessile. From a distance the premature shoots look like large
double red flowers inserted in the dark green of the non-infected Cowberry bush.
The shoots which develop prematurely on the shrubs of the Bog Whortleberry
(Vaccinium uliginosum) by the action of Exobasidium Vaccinii are often met
with in alpine regions, and are even more noticeable on account of their fiery-red



ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 527

colour. The Bearberry (Arctostaphylos Uva-ursi), Ledum palustre, and the Marsh
Andromeda (Andromeda polifolia) are subject to similar metamorphoses at the
hands of Exobasidium Vaccinii, so that Vaccinium Vitis-Idcea may be regarded
AS typical of them.

When the shoots of the larger shrubs or trees are metamorphosed by parasitic
Fungi attacking their branches, we have the formation of the structures popularly
termed Witches' brooms, which were mentioned at the beginning of this chapter.
The stimulus necessary for their formation is afforded in different plants by
different parasites; on Barberry bushes (Berberis vulgaris) by ^cidium Magel-
hcenicum (to be distinguished from the common &. berberidis), on the Gray
Alder (Alnus incana) by Exoascus epiphyllus, on the Hornbeam (Carpinus
Betulus) by Exoascus Carpini, on the Bullace (Prunus insititia) by Exoascus
insititice, on other species of the genus Prunus by Exoascus Cerasi, on the
Birch (Betula verrucosa) by Exoascus turgidus, on the Weymouth Pine (Pinus
Strobus) by Peridermium Strobi, and on the Silver Fir (Abies pectinata) by
^Ecidium elatinum. Witches' brooms also occur on the Mastic tree (Pistacia
Lentiscus), and on Beeches, Pines, Larches, Spruce Firs, &c., although hitherto
we have not been able to ascertain definitely what parasitic Fungi are the
<jause in these cases. The Witches' broom of the Silver Fir has been selected
.and figured (see fig. 359) as a type of these peculiar structures. It always
grows on one of the horizontally projecting lateral branches of the Fir, and
raises its erect or curved twigs from the upper side, resembling, as it were, an
epiphyte growing on the bark of the horizontal bough. The twigs are grouped
in whorls and not in two rows, as usually happens in the lateral shoots of the
Silver Fir. They are all shortened and thickened, and remarkably soft and
pliable, because the cortical parenchyma has become spongy and the wood is
only slightly developed. The buds, which in healthy tissue are egg-shaped,
-are almost spherical here. As in other instances of hypertrophied plant-members,
we have a precocious development, a so-called "prolepsis", in these Witches'
brooms. The buds swell earlier and unfold earlier than those of healthy twigs.
The leaves remain short, yellow, somewhat crumpled, and fall off when a year
old, while those of normal twigs are long, linear, straight, dark green on the
upper side, and remain in position from 6-8 years. The growth of the twig
is restricted; it dies off in a few years, and then, inserted on the dark green
branches of the Silver Fir, remain the dry, bristling brooms, whose appearance
has stimulated the imagination of the peasantry and given rise to the superstitions
alluded to at the beginning of this chapter.

ALTERATION OF FORM BY GALL-PRODUCING INSECTS.

Certain members of the Arachnoidea, Diptera, and Hymenoptera, which
attack and penetrate the tissues of living plants and incite the formation of
peculiar excrescences, are known as gall-mites, gall-gnats, and gall-wasps. The



528 ALTERATION OF FORM BY GALL-PRODUCING INSECTS.

growths, like small rosy-cheeked apples, which occur on the foliage of Oaks,
popularly known as "oak-apples", are amongst the best known. The terms
" gall " and " gall-apple " were used by writers in the sixteenth century, and (like
the Old English word galle, the French galle, and the Italian galla) are derived
from the Latin word galla, used for these outgrowths by Pliny in his Natural
History. The sixteenth-century writers distinguish between "gall-nuts" and
"gall-apples", meaning by the former the small hard outgrowths on the leaves
of Beech-trees. Afterwards the word gall was used for all the outgrowths
produced by animals on green living plants. More than that the hypertrophies
described in the preceding chapter, produced in green host-plants by the various
families of Fungi, are also included under the term. It has been proposed recently
to substitute the word cecidium for gall, and to distinguish the excrescences as
myco-cecidia, nemato-cecidia, phyto-cecidia, diptero-cecidia, &c., according as they
owe their origin to Fungi, Thread-worms (Nematodes), Gall-mites (Phytoptus),
Gnats (Diptera), &c. A systematic classification of this sort, on the lines of
the classification of animals, might be of use to Zoologists, but to the Botanist its
value is only secondary. He must, as in other similar cases, keep to morphology as
the primary ground of classification, and has to arrange the structures according
to their agreement in development. Moreover, in a general review, it is necessary
to consider whether a whole group of plant-organs or one alone undergoes metamor-
phosis; and the starting-point of the outgrowth must also be ascertained; i.e.
whether it is the foliage-leaves, floral -leaves, stems, or root -structures, &c., which
are the head- quarters of the excrescence.

When the gall originating as the nest or temporary habitation of a single
animal or colony of animals is limited to a single plant organ it is said to be
simple; if, on the other hand, several plant organs are concerned in its production
it is said to be compound.

Simple galls may, for convenience of description, be divided into (1) Felt-
galls, (2) Mantle-galls, and (3) Solid galls. The Felt-galls are chiefly due to
hypertrophied epidermal cells growing out into hairy coverings of various sorts
and shapes; Mantle and Solid galls, however, are rather more complicated.
In both cases insects are present in swellings of various descriptions, but there
is this essential distinction: The Mantle-gall is a hollow structure which,
though it may arise in various ways and assume a multiplicity of forms, always
has a portion of the surface of the affected organ for its lining in other words,
it is a chamber formed by hypertrophied growth around the place occupied
by the insect. In the Solid gall, on the other hand, some spot is pierced by an
insect and the eggs deposited in the tissues (not on the surface), the punctured
spot forms a swelling with the larva inside, but the lining of the chamber is



Online LibraryAnton Kerner von MarilaunThe natural history of plants, their forms, growth, reproduction, and distribution: from the German of Anton Kerner von Marilaun (Volume 2) → online text (page 63 of 128)