Heinrich Schenck Eduard Strasburger.

A text-book of botany online

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The sexually produced offspring, on the other hand, endowed
with the properties of the father, can never be identical with the
mother-plant, but possess the properties of both parents. When these
are divergent they frequently play very different parts in the
descendants, some (dominant) characters appearing conspicuously,
while others (recessive characters) become less marked or remain
completely latent. In this way the descendants do not exhibit a
uniform mean between the parents, but some may resemble the father,
others the mother. These relations determine the character of the



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300 BOTANY part t

sexually produced descendants (cf. p. 313 ff.). Variations appearing
in single individuals will, unless they are of an absolutely dominating
character, become modified and ultimately lost by crossing with
ordinary individuals. In such a case sexual reproduction tends to
maintain the constancy of the species. In other cases, as when one
parent possesses new and dominant characters or when both parents
tend to vary in the same direction, the deviation from the ancestral
form may be maintained or increased by sexual reproduction.

The great tendency to variation commonly exhibited by hybrids
(p. 313) illustrates how the equilibrium of the complex of properties
of a sexually produced individual is affected by divergent parental
tendencies. But, even as a result of ordinary fertilisation, not only
small and readily disappearing variations (fluctuating variations) but
sometimes more striking ones occur, in which the offspring differs so
strongly from the parents in characters, which can be inherited, that it
appears to be a new species or sub-species. Of such petites espkes of
Draba vema some two hundred are known. In such sudden variations
(the occurrence of which v. Kolliker, and with him Korschinsky,
term heterogenesis, while de Vries more recently calls it mutation)
these authors seek the starting-points of the origin of new species.
This would occur when a particular species passes, from unknown
causes, into a period of mutation such as de Vries demonstrated
experimentally in Oenothera Lamarckiana. Korschinsky collected a
number of historical examples of heterogenesis, of which Chdidonium
Idciniatum, which appeared in a garden at Heidelberg in 1590, and
Capsella Heegeri, SoLMS, which only recently appeared, will serve as
examples Q^).

The fluctuating variations which largely determine the valuable characters of
economic plants {e.g. the high percentage of sugar in the Sugar Beet) are in con*
trast to the mutations not fixed on inheritance. Careful and continued selection of
the varying progeny is thus necessary to maintain the required standard of the race.

The experience of cultivators and the recent experimental work
on lower plants carried out by Klebs show that different groupings
of the internal and external conditions of life favour reproduction
and ordinary growth respectively. In fact growth and reproduction
frequently though not always appear to be mutually exclusive.

Vegetative Reproduction

Vegetative reproduction, the .purely quantitative character of
which as a mere process of multiplication has been emphasised, exists
generally throughout the vegetable kingdom, and but few plants, e.g,
some of the Conifers and Palms, are altogether devoid of it. Mention
has already been made in considering artificial propagation that,
from the separate parts or single cells, or even from the naked
protoplasts (Siphoneae) of many plants, the regeneration of a new and



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PHYSIOLOGY 301



perfect individual may ensue. In vegetative reproduction the process
is similar except that the separation of the part from the parent plant
is an organic one, occurring in the natural course of development.
The vegetative form of reproduction is manifested in various aspects,
and may be distinguished as a multiplication by means of multicellular
vegetative bodies (buds, gemmae), or by single cells (spores).

Multiplication by Multicellular Vegetative Bodies (Budding)
often consists merely in the separation of lateral shoots, or in a
division of a single plant into several. In this way the lateral
shoots of Azolla, through the death and disruption of the older
parts of the parent axis, become separated from one another and
continue their growth as independent plants ; similarly, separate
plants originate from the vegetative body of the Duckweed (Lemna).

Multiplication by stolons, rhizomes, and tubers results in a similar
formation of independently existing plants. As may be seen in the
Strawberry, the Bugle {Ajuga reptans) and numerous other plants,
the shoots produced from many of the axillary buds of the widely
outstretched stolons take root and form new plants. In cases where
the runners themselves eventually die, the parent plant becomes finally
surrounded by a colony of entirely independent plants. Instead
of forming runners, the single tuber may divide {Corydalis solida),
and in this way give rise to two, four, or more new tubers. New
bulbs are produced in the leaf-axils of the bud-scales of bulbs, while
brood buds (bulbils, gemmae) are frequently developed on aerial
vegetative organs.

Bulbils are found on the inflorescence in the place of the flowers in many species
of Allium^ in the grass Poa hulbifera, and also in Polygonum viviparum. In
Lilium bulbi/erumy Dentaria hulbifera^ etc., the bulbs in the axils of the leaves are
specially constructed with a view to detachment from the parent plant (Fig. 22).
The swollen leaves contain reserve food material, and frequently develop roots before
falling from the plant. In Ranunculus Ficaria the roots of the axillary buds are
full of reserve food material, and resemble grains of corn. "When the plant dies the
bulbils remain on the ground, and have given rise to the fable of showers of grain.
Bulbils or gemmae are met with also among the Mosses, Liverworts, and Ferns. The
winter buds of many water plants {Hydrocharis, Utricularia, Lemna, etc. ) have a
peculiar biological significance. They are formed in the autimin, and sink to the
bottom of the water ; in the succeeding spring they rise to the surface and form
new plants.

By vegetative multiplication higher plants can annually give rise to individuals
which are strong and capable of flowering and fruiting. The seedlings of such
plants, on the other hand, often require to grow for several years before the
capacity of sexual reproduction is attained (bulbous plants. Hop, etc.).

In addition to the instances just cited, in which the vegetative reproductive
bodies take their origin from jwints where lateral shoots are normally formed, they
may also appear in places where no shoots are normally developed. Thus tlie
adventitious formations often found on leaves, particularly on tlie leaf-blades,
serve the purpose of reproduction. Just as the leaves of Beganm, Drosent, etc.,



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302 BOTAXY part i

al'ter they liavc been cut off, are able to give rise to new plants, in other cases the
leaves jwssess this power while still growing on tlie parent plant. Some ferns
afford si)eeially characteristic examples of this {Asplcnium decussatum, A.
Fabianuvij A. bulbiferunij A. viviparum) ; adventitious buds are produced on their
laminie, developing into small rooted plants, which then fall off and complete their
development (Fig. 239). The adventitious buds of Cystopleris bulbifera take the
form of bulbils with small swollen leaves. Adventitious plantlets are frequently
formed also on the leaves of Cardaniinc pratcnnSj and Cardamine anuira manifests
a similar tendency. One of the best-known examples of such adventitious forma-
tions is afforded by the leaves of the tropical Bryophtjllum, in the marginal indenta-
tions of which the brood plantlets develoj) in gi'eat numbers. Even the force of the
wind is sufficient to detach the leaflets of Bryophyllum from the i)lant. Gemmae
are abundantly produced on the thallus of many He|)aticae {Marchantiu, Lunularin),
and by their continuous growth the gemma cups (Figs. 381, 382) are always kept
well filled.

One of the most interesting cases of adventitious budding is the formation of




Fhj. •l',VX-—A>'i>lpn,laui Fatnanum. A yoiiiii^ jjlant (2*), « itli leaves and nxitn {W),
1ms siuiiii^' from the leat(.U) of tin' o1«I»t i)Iaiit.

adventitious embryos in ovules ; in most instances this leads to I'olyembryony,
i.e. the presence of several embryos within the one seed. In addition to cases in
which the additional embryos cannot be strictly regarded as adventitious but
arise by fusion of ovules, by division of the nucellus within the integments, or by
the existence of a number of embryo-sacs in one nucellus, normal ovules with only
one embryo-sac may ju-oduce a number of embryos. In the latter case the embryo-
sac may in rare instances [SaiUalum, Siiinimjia) contain two egg-cells, or a
dichotomy of the embryo or its suspensor may occur. As a rule tlie condition is
due to adjoining vegetative cells growing into the embryo-sac and there develop-
ing like sexually produced embryos. These adventitious embryos, which were
lirst discovered by Strasburger, usually originate from cells of the nucellus {Funkia
ovata (Fig. 240), Euonymas lati/olia, Citrus auraatimn, Nuthoscordon fraijrans,
Maayifera indica, ('ocbhoyyiie illicifuliay Clusia alba, Opantia imlaarc, Euphorbia
dulcis, Culchicum aidu/iuiak) more rarely from the inner integuments {Alliuia
vdorum). In many other j>lants {diictuia, Ulmus) embryos may develop from the
synergidae or the antipodal cells. At the same time the egg-cell previously
existing in the embryo-sac is able to continue its development after fertilisation,
but is usually prevented from so doing by the adventitious embryos. The seeds
in such cases would no longer contain the products of sexual reproduction, but



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SECT. II



PHYSIOLOGY



303




would be degradtMl to organs of vegetative multiplication. The adventitious
germs in the jwly embryonic seed are, however, so far dei)endent upon sexual
reproduction, that for the most i>ai't they only attain their development in case
]K)llination has previously taken place ; but in CoeleboffynCy one of the Australian
Kuphorbiaceae, of which usually only female sjjecimens are found in cultivation,
and in Balanophora clontjata and Elatosteitia acuminatum according to Treib,
and Bcil. fjlobosa according to Lotsv, the adventitious germs develop without the
stimulus of fertilisation. These plants accordingly afford examples of AroGAMY,
or of the substitution of a Au?getative for a sexual mode of re])roduction, such as
occurs in certain ferns.

In the aiK)gamous ferns vegetatively produced plants arise on the prothallus
in the j^wsition of the sexual organs. This is found in different degrees in
Athyrlum Jilix femiiia, var. cristatay As-
jtidium falaitum^ l^odea africaiuit Ftcris
eretica, and in Nephnxlium pscudo-mas.
var. polydaetyla. In the latter exajuples
the sexual organs are no longer formed,
although the young plants arise, by a
vegetative process of budding, from
exactly the same part of the prothallium
where the archegonia would have been
developed. Faumkii, Moore, and Digby
have shown that in Xcphroilium the origin
of theapogamousbud is preceded by fusion
of nuclei of vegetative cells of the pro-
thallus. In the case of A»piiilum Jili.r
nias. var. cristatum, etc., the apogamy
seems to have resulted from cultivation.
In a broad sense the development of
bulbils in the place of flowers, in the
species of Allium^ might be considered
as an exami)le of apogamy (^**).

The cases of parthenogenesis de-
scribed in recent years among the higher
plants (Compositae, Akhemilla^ Thalic-
trunif Bryonia ?) may be regarded as
examples of a further peculiar ty})e of
aiwgamy. The ovum develops into an embryo without fertilisation, but since
the reduction division has been omitted from the processes leading to its differ-
entiation the egg cell has in these cases lost the characteristics of a female
sexual cell and corresponds to a purely vegetative cell. The same will jjrobably
be found to be the case in Wikstroemia indka, Ficus hirta^ and Cham crinita which
are also parthenogenetic {^^).

In some cryptogamic plants (Marsilia, Saprolegnia) as among
lower animals, true parthenogenesis occurs. LoEB found that solu-
tions which withdrew water (MgCU, other salts, sugar, urea) could
stimulate the ovum to parthenogenetic development. AVinkler used
extractives derived from the sperm, while Nathansohn showed that
sometimes a rise of temperature could interrupt the resting state of
an ovum awaiting fertilisation and lead to a parthenogenetic develop-






r\



/



j:t



Fio. iMtK— VoK'^tative fomiHtioii of einhryos in
Funkia om/a {llosta coernlea) by the buddiii}^
of thf miol»»us ; n, iiucelliis with cells in
jirwoss of fonning tht* nidimeut* (ae) of th«
adveiititioii.H »'nihryo« ; S, syii»Tjrida? : K, pkj{-
cell, in the lower li^ure developinj; into a sexii-
ully-prodiired enibrjo ; U, inner inte^iument.



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304 BOTANY part i



ment. Jickeli regards the common feature of the various causes
which lead to parthenogenesis to be that they are injurious influences,
the effect of which, however, must not be carried too far (^®^).

Vegetative Multiplication by Single Cells (Spores). — As in the case of multi-
cellular vegetative bodies, multijilication can be effected also through the separa-
tion of single cells. Strictly speaking, this luanner of multiplication actually
takes place whenever a division of the vegetative body occurs in unicellular
Bacteria, Fungi, and Algae. Cells which serve the purpose of vegetative reproduc-
tion, and have a special form and method of development (spores, conidia, swarm-
spores, gemmae), are first met with in the higher Cryptogams. They are frequently
formed in special organs or receptacles (sporangia, conidiophores, fruit bodies).

As a Fern-plaut occasionally arises directly from the tissue of the prothallus
without the intervention of the sexual act, so also si)ore formation is occasionally
omitted, and the prothallus arises vegetatively from the leaf of the sporophyte
(apospory in \SLneties of A thy r turn ^ AspUiium, Aspleniumdiinorphmny Nephrodium
pseudO'Vms. var. cnstata apospora) (^**ft).

Sexual RepFoduetion

For the purpose of sexual reproduction two kinds of cells, male
and female, are produced. Although neither alone is as a rule
capable of development, the actual reproductive body is formed by
the fusion into one cell of two such sexually differentiated cells.
The elements of the male and female nuclei remain for a longer or
shorter time distinguishable in the nucleus resulting from their
fusion (^^).

In fertilisation, as a rule, two uninucleate cells fuse, even when the vegetative
protoplasts are multinucleate. In Albuyo BHtif A. portul^cae and A. tragopogoniSj
however, Stevens found that the numerous nuclei of the ovum fused with an equal
number of sperm nuclei. Probably other cases of this kind may be found.

The further development of non-nucleated fragments of the eg^ when united
with a spermatozoid has been shown to occur among animals by O. and R. Hrrtwio,
BovEiir, and Delage. This phenomenon which is termed meuogony has also been
observed by Winkleii in Cystoseira barhata, one of the Fucaceae Q'^%

In connection with parthenogenesis it was noted above that the incapability
of further development which characterises the unfertilised ovum may be over-
come by other means than copulation with the male cell ; Elebs had already
shown this to be the case with the gametes of certain Algae. It is thus necessary
to distinguish in fertilisation between a stimulus, which removes the arrest laid
on the further development of the ovum, and the cell fusion, which influences
the nature of the resulting organism (amphimixis). The two influences are united
in the case of natural fertilisation ("*).

As it is thus necessary in sexual reproduction not only to provide
for the production of male and female cells, but also to ensure their
union, it becomes at once evident that, for sexual reproduction, the
organs must have a different structure than if they were designed
solely for vegetative activity. The sexual organs accordingly often
exhibit a special and peculiar form, and differ materially in appear-
ance from the vegetative parts of a plant.



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PHYSIOLOGY 305



The Union of Sexual Cells (Fertilisation). — Leaving out of con-
sideration the necessary contributory external contrivances, fertilisa-
tion is accomplished by means of a cbemotactic or chemotropic stimulus
(pp. 266, 286). It is generally the non-motile egg-cells or the female
sexual organ which exert an attractive influence upon the motile male
cells. When, however, there is no dift'erence in the external form of
the male and female cells, then both are usually motile, and the attrac-
tion seems to be exerted mutually. This is probably the case with
the motile and externally similar sexual cells (gametes) of the lower
Cryptogams, particularly of the Algae (Fig. 97). In the conjugation
of the Conjugatae, however, although both sexual cells are externally
alike, one cell alone is usually motile, and passes through the connect-
ing canal to the other. The capacity of the male cells for independent
movement is common to most Algae, with the exception of the
Florideae, in which the non-ciliated male cells are passively conveyed
to the female organ by the water. Throughout the whole group of
the higher Cryptogams, and in a few Gymnosperms, the male cells are
motile spermatozoids, capable of seeking out the non-motile egg-cells
concealed within the archegonia. But in the sexually differentiated
Fungi the male substance usually remains enclosed in special hyphae
which press themselves close against the female organs, and, by the
perforation of the intervening cell wall, the fusion of their contents
is rendered possible. The fertilisation of the Phanerogams is accom-
panied by a perforation of the intervening cell walls similar to that
which occurs in the Fungi. In this case the male cell is enclosed
within the microspore (pollen grain) ; the female, as a naked egg-cell,
is included in the megaspore (embryo-sac), which in turn lies in the
ovule, and in the Angiosperms the ovule is again enclosed within the
ovary. The double-walled pollen grains possess no independent power
of movement, but are conveyed to the female sexual organs by the
assistance of external agencies (animals, currents of air or water).
The pollen grain then grows out into a tube which is acted upon by
chemotropic (including hydrotropic and aerotropic) influences, and
grows like a fungal filament through the tissues of the ovary and
ovule until it penetrates to the egg-cell in the embryo-sac ; the
union of the sexual cells is then easily eflfected (Fig. 99).

Recent researches have shown that in addition to the fusion of the generative
cell with the ovum the second generative cell frequently fuses with the nucleus of
the embryo-sac. This has been termed double-feutilisation. The endosperm,
which arises from the resulting nucleus, is thus, like the embryo itself, a product of
fusion. This explains the hybrid character of the endosperm in the phenomenon of
XKSIA. In these cases characters of the i>ollen-parent appear in the ripening seed
and not, as is usual, only in the descendants of the union. This is especially well
.shown in the maize when, e.g. blue- and yellow-fruited races or a race tlie fruits of
which contain sugar and one in which they contain starch are interbred. It is
uncertain what significance is to be attached to this so-called double fertilisation ;

X



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306 BOTANY



possibly the ex])lanation is that the embryo-sac nucleus, as a sister micleus to that
of the ovum, exercises a similar cliemotactic influence, and thus attracts the male
generative cell, the fusion being of secondary importance since the endosi>erm is
destined to be used as food by the embryo (^^^).

AVhile one healthy pollen grain should be able to fertilise one
healthy ovule, experience shows that better results follow from more
liberal pollination of the stigma. This depends, according to
CoRKENS, on the fact that not every pollen grain (and not every
ovule) is good. Thus in MirabUis jalapa for each fertile pollen grain
four are found infertile, and for every three good ovules there is one
bad one. For MirabUis longijlora the corresponding ratios are 1 : 3
and 1 : 1 {^^%

To render certain the accomplishment of this pollination, or
conveyance of the pollen to the female sexual organs, special and
often complicated contrivances are made use of by the different
Phanerogams, according to the means of conveyance upon which they
are dependent.

Plants, the pollen of which is carried by wind, are designated
ANEMOPHILOUS. As this method of conveyance depends upon the
chance of wind direction, the production of an enormous amount of
pollen characterises wind-fertilised plants. According to Holden a
medium-sized plant of maize produces about 50,000,000 pollen grains.

Such enormous quantities of pollen are often taken up from pine forests by the
wind that clouds of pollen fill the air. The surface of Lake Constance in spring is
so thickly covered with pollen that it is coloured yellow ('* the lake* blooms, " it is
then said), and in the Norwegian fiords, at a depth of 200 fathoms, the pollen of
Conifers, according to F. C. Noll, forms for a time the principal nourishment of
a Khizopod (Saccainina).

The male flowers of such anemophilous plants are accordingly either freely
exposed to the wind in Catkins (Coniferae, Querciflorae), or the versatile anthers,
as in the Grasses, depend from long, lightly-swaying filaments. The pollen grains
themselves do not stick together but escape from the opened anthers in the form of
fine powder. The pollen grains of many Conifers are rendered extremely buoyant
and easy of conveyance by the wind by two sac-like protrusions of the exine. In
some anemo})hilous plants the 2)ollen is discharged by the sudden extension of the
filaments, previously rolled up in the bud (Urticaceae, e.g. Pilca)^ or by the hygro-
scopic tension of the anthers. The female organs are also often specially adapted
for the attachment of the 2X)llen thus floating in the air. The stigmas either spread
out like a brush (Corylus), or are finely feathered or provided with hairs (Grasses,
Walnut), or drawn out into long threads (Indian Corn). In the Conifers, with
freely exposed ovules, the grains of pollen are caught and retained, in a drop of
fluid exuded from the micropyle, into which they are gradually drawn as the fluid
dries up. In other Conifers whose ovules are concealed in the cone of the female
inflorescence, processes of the integument catch the jwllen and conduct it to the
sticky oi)ening of the young ovules.

For the fertilisation of the higher plants, the presence of water is
not so essential as it is for most Cryptogams. Only a few submerged



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»ECT. II PHYSIOLOGY 307

Phanerogams make use of the agency of water for effecting their
pollination, and are, on that account, termed hydrophilous plants.

The pollen of the submerged Zostera exhibits certain peculiaritieSj distinctly
referable to the necessity of effecting fertilisation under water. It does not form
round grains, but in their place elongated thread-like filaments devoid of an exine,
which, as they have the same specific weight as the surrounding water, are easily
set in motion by the slightest currents, and are thus brought into contact with the
stigmas. In the case of the submerged water plants, Fallisneria, Elodea, and
species of EnhaluSy found in the Indian Ocean, the pollination is accomplished on
the surface of the water. Thus, for example, the male flowers of VallisTieria, after
separating from the parent plant, rise to the surface of the water, where they open
and float like little boats to the female flowers, which, by the elongation of their
spirally coiled flower-stalks, ascend, at the same time, to the surface of the water,
only to become again submerged after fertilisation C^*).

In the great majority of Phanerogams pollination is effected by
means of animals. By enticing in various ways insects, birds, bats,
or snails, plants are enabled not only to utilise the transporting
pK>wer but also the intelligence of animals in the service of pollen-



Online LibraryHeinrich Schenck Eduard StrasburgerA text-book of botany → online text (page 33 of 70)