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Growth.

As far as observation has extended, all cavity-formation
during primary growth is in its beginning schizogenous. But
this splitting apart of cells before collapse of cells ensues may
be on the one hand very extensive, while on the other hand
it may go no farther than the formation of small intercellular
spaces. The process has been described for several plants by
Frank ^, Trdcul ^ and others, and a general review of the matter
is given by De Bary ^ The cause for the appearance of these
schizogenetic clefts is to be found in the rapid extension of the
peripheral zones of tissue opposed to the more slowly extend-
ing or wholly non-extending tissue in the localities where the
clefts arise.

Using for illustration only those plants which have been
the subjects of my own observation, the leaves of Allium
Cepa, the peduncle of Taraxacum Dens-leonis, the upper part
of the stem of Vicia Faba, and the stem of Caltka palustris,
may be cited as examples in which the splitting apart of cells
during primary extension gives rise to central cavities of
considerable size before any cells die.

* Frank, Beitr. ztir Pflanzenphysiologie. Leipzig, 1868, p. 145.

« Ti^cul, Ann. Sci. Nat. 4^ ser. I, p. 166.

' De Bary, Vcrglcichcndc Anatoniie, %% 51, 53.



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of Lysigenaus Cavity-formation. 405

In Allium Cepa, while the leaf retains in cross-section the
semilunar form, the parenchymatous network of central cells
is wholly living. When the leaf however begins to assume
its inflated form, accompanied by the great growth in size of
the peripheral cells, the central network is broken by the
splitting of cell-walls, so that there arise plates of cells pro-
jecting freely into the schizogenous cavity and attached along
only one border of the plate. These cells, however, at this
time contain a good supply of protoplasm and still live for
a period, but at last collapse, adding thus lysigenously to the
size of the already existing schizogenous cavity.

What has been stated for Allium is true in general also for
a great many other plants, especially for the peduncle of
Taraxacum Dens-leonis, the stem of Caltha palustris^ and the
upper internodes of Vicia Faba. In all of these, the cells
where the cavity is forming suffer a considerable separation
into plates or strands, so that a large part of the cell-surface is
exposed to the space of the cleft before any cells die.

But this procedure is not the one found in all plant-organs
that form lysigenous cavities during primary growth. All but
the lowermost internodes of the stems of Cucurbita Pepo^
Dahlia variabilis^ Archangelica sativa, Myrrhis odorata^ Melu
anthus major ^ Ricinus communis^ and many other plants, pro-
ceed no farther in the schizogenous formation of the central
cavity than to produce ordinary intercellular spaces, or at
most to increase the size of such spaces to very small clefts,
before the collapse of cells begina Thus, for instance, though
during primary growth the schizogenous formation of cavity
in the pith of Cucurbita Pepo and Melianthus major is at first
relatively small, the cavity becomes very large by the collapse
of a few central cells occurring at a relatively earlier period
than in the first group of plants cited, and by the subsequent
great extension of the cells lying just outside those that
collapse.

Between these two extremes, in the one of which the
schizogenous process is of large extent, and in the other goes
at first but little farther than the formation of ordinary inter-



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4o6 N€wcombe.^-On the Cause and Condiiions

cellular spaces, there are all gradations shown by different
plants. The essential difference of the two processes resides
in the unlike development of the pith. In the first caae the
central and peripheral parts of the pith mature at about the
same time, so that the schizogenous cleft is large and is
followed by the rapid collapse of cells throughout. In the
second case the central part of the pith matures first, while the
peripheral part continues to grow, so that the first schizo-
genous clefts, though small, are followed by the collapse of
the few surrounding cells, and the cavity increases in size by
the continued outward movement of the peripheral zone of
pith and by die collapse of a few cells bounding the already
existing cavity. Considered in toto^ the schizogenous factor
may be much larger in this latter case than in the former,
though the initial clefts are much smaller in the latter.

2. LySIGENOUS CA\nTY-FORMATION SUBSEQUENT TO

Primary Extension.

Although with the cessation of primary extension the
increase of tension between the pith and the more peripheral
tissues, due to displacement of the peripheral zones, goes no
farther, there are cases found in which cavities, though not
appearing till after primary growth is complete, nevertheless
b^n schizogenously. In illustration of this statement may
be mentioned the cavities in the pith of Althaea taurinensis^
Silene viridiflora^ and Eryngium planum. The cavities in
these plants, due mostly to the collapse of tissue-elements,
show themselves in their initial stage as small clefts between
living cells. In the particular plants just named the cavities
appear soon after the completion of primary growth; the
schizogenous clefts are, however, formed during primary
growth, but the adjoining cells live for some time afterward,
that is, till secondary growth has b^^n.

Schizogenous clefts may, however, appear in the pith a long
time after the surrounding zones of tissue have ceased to
travel outward from the centre of the stem. We have only to



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of Lysigenous Cavity-formation, 407

think of the pith-cells as diminishing the amount of their
turgor while the outward pull of the more peripheral zone
remains constant, to understand how, by the consequent
inclination of the less turgid cells to decrease in size, the
tension between such tissues is increased, and ensuing splits
may follow before any cells die. In this manner are to be
explained the clefts found in the pith of the lower intemodes
of many plants, which mark the beginning of cavity-formation
a considerable period after primary extension has ended.

In by far the largest number of plants in which lysigenous
cavities appear in the pith after primary growth has ended,
there is a collapse of cells before their separation by more
than ordinary intercellular spaces. This is the case with the
rhizome of Triticum repens^ the cavity not being present for
some weeks after the full diameter of the rhizome has been
attained. The cells collapse and may split apart in so doing.
The lower intemodes of Lamium garganicum^ Urtica dioica^
Dahlia variabilis, Archangelica saliva, Vicia Faba, Ricinus
communis^ and many other plants, form their cavities in the
pith by the shrinking and collapse of cells, without showing
previous separation of cells.

In the foregoing examples there is a continuous cavity
formed through each internode. Another group of plants in
which the cavity is interrupted by diaphragms, is represented
by Juglans^ Pterocarya^ and Forsylhia. The formation of
these diaphragms has been studied and described by Kassner^.
The cells of the pith, none of which die for weeks after
secondary growth has b^fun, begin to contract and separate
in horizontal planes some distance from one another. Both
the radial and longitudinal pull on the pith-cells is thus
relieved, partially at least, so that the cells forming a hori-
zontal plate midway between the planes where separation and
collapse of cells has begfun, are not pulled apart. Cells both
above and below this plate of tissue are drawn nearer and
nearer to it, till at last there remains only a comparatively

' Kassner, Ueber das Mark einiger Holzpflanzen. Inaug. Diss. Breslau, 1884.



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4o8 Newcombe. — On the Cause and Conditions

thin diaphragm of dead, shrivelled cells with a rather wide
cavity on each side.

From this condition of the pith, in which some cells retain
their position, we can pass on to that in Sambucus and
Helianthus^ in which all of the cells, though dead, retain their
position, and the dead tissue has in it only the ordinary inter-
cellular spaces. In these two plants, as is well known, the
pith dies during the first season's growth, but not till a wide
zone of secondary formation has arisen. Details need not be
given here of various other plants in which the death of the
pith takes place one or more years after the first season, and
it need only be mentioned that in such cases the pith some-
times collapses^ while at other times the cell-skeletons retain
their primary position. The work of Gris^ may be consulted
for the age at which the pith of various plants dies.

3. Lysigenous Cavity-formation either previous or
subsequent to the cessation of primary growth.

It has been shown that nearly the same appearances accom-
pany the formation of cavity during and subsequently to
primary extension. In both stages of growth the cell-walls
may be split apart, and intercellular clefts be formed, before
any cells die ; in both, the collapse of cells may begin before
the cells are separated by large clefts. It remains to be
stated that there are plants of the same species in which, in
corresponding intemodes, the cavity is in one individual formed
during primary growth, while in another it appears at a longer
or shorter time after the cessation of primary growth.

Urtica dioka and Dahlia variabilis verify the truth of the
last statement. A strong plant of the former species exposes
above the ground but five or six intemodes before a cavity,
appears in the third internode, elongation being there incom-
plete. Slender plants, however, may grow to a height of
twelve intemodes above the soil and still show no cavity

' Gris, Sur la moelle des plantes lignenses. Ann. Sci. Nat. 5* s^r. XIV.



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* of Lysigenous 'Cavity-formation. 409

anywhere, though the lowest half-dozen internodes are fully
elongated. In the average plant of Dahlia the cavity in the
pith will be found present in all the internodes except the one
nearest the ground before elongation has ended. Slender
plants grow to a height of seven or more internodes, with the
lowest five fully elongated, before a cavity appears.

The histological differences attending the formation of such
cavities in corresponding internodes before and subsequently
to primary extension are not great. When the cavity appears
while radial displacement of the vascular ring is progressing,
the clefts between the cells become larger before cells collapse
than in the other case, where primary extension has ended
before the cavity forms. Moreover, in cross-section the cells
of the pith are easily seen to be smaller in those slender stems
in which the pith lives on into the period of secondary forma-
tion, than in the individuals in which it dies earlier. There is
also apparent in the vascular and cortical zones of the thick
stems a greater tangential expansion relatively to the size of
the pith than in the slender stems. In other words, the primary
radial and tangential extension of cortex, vascular zone, and
pith have, in the slender plants, more nearly coincided in time
than in the thick ones. The length of the internodes, however,
is as great in the slender as in the stronger plants.

When the foregoing facts are properly arranged it will be
seen, I believe, that the formation of cavity during primary
extension is to be traced ultimately to the same cause as the
formation of cavity or the death of the pith subsequently to
the cessation of primary extension, that is, to the fact that the
cells concerned have reached the stage where, without the pull
of the more peripheral tissues, they would soon die. That,
however, the life-period of such cells would be slightly
prolonged did this forcible tearing not occur will appear from
what follows.

We have, then, a series in the formation of cavity, the one
extreme of which falls in the period of primary extension and
is represented by the axial tissue of the leaves of Allium Cepa
and of the stem ol Dahlia^ while the other extreme falls in the

Ff



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4IO Newcombe. — On the Cause and Conditions

period of secondary growth and is represented by the pith of
Juglans and of Sambucus.

The immediate and apparent cause of the formation of
cavity in such cases as those cited for primary growth, lies in
the inability of the central mass of tissue to keep pace in
growth with the more peripheral zones. If the peripheral
zones continue to expand for a considerable period after the
central mass has ceased growing, or if the peripheral part
grows for a considerable time much more rapidly than the
central part can extend, there will be formed a large central
cavity during primary growth. If, however, the peripheral
tissue ends its primary extension soon after that of the central
mass, the cavity formed during primary growth will be small,
but the cavity will subsequently continue to enlarge by the
lysigenous process.

The latter of the two cases just cited is that of some of the
lower intemodes of many plants, including Vicia Faba^ Dahlia^
and Ricinusy and passes insensibly into the condition in which
the primary extension of the pith persists to the completion of
primary extension in vascular zone and cortex. In the latter
case the cavity-formation may begin soon after the ending of
primary extension, or the pith may live on for weeks or years.
The fact that in some plants, as pointed out for Urtica dioica
and Dahlia^ the cavity appears before or subsequently to the
completion of primary growth according to the amount of
primary extension of vascular zone and cortex relative to that
of the pith, indicates that the life of the pith-cells is shortened
by the tearing apart to which they are subjected in the one
case, and indicates in the other case that the cells would not
live much longer if not subjected to the tearing. But since,
as has been pointed out by Kraus^, the stretching of cells due
to turgor increases as they pass from the embryonal condition
and decreases as they assume their permanent condition, and
since the parenchyma, with thin walls of cellulose, such as
generally makes up the pith, must contract when the force of

> Kraus, Die Gewcbespannung des Stammes «nd ihrc Folgen. Bot Zcitung,
1867, p. 105.



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of Lydgenous Cavity-formation. 411

turgor is withdrawn, it follows that in those stems in which
the extension of the pith, or, better, the positive tension of the
pith, ceases with the end of primary growth, there must be
a contraction of the pith as the latter loses more and more of
its turgor. When the n^jative tension thus called forth is
considerable, schizogenous clefts may precede the collapse of
cells.

In those plants like Sambucus and Helianthus tuberosus, in
which the pith dies during secondary growth without col-
lapsing, it is probable, as found true by Kraus for the two
plants mentioned, that the pith is in the condition of positive
tension for some time after the beginning of secondary growth.
When the cells lose their turgor, the force of contraction is
not sufficient to separate them.

4. Effect of Prevention of Tension.

Many years ago Sachs ^ found that leaves of Allium Cepa
grown in the dark were not hollow ; he did not, however,
describe the difference in the histology of normal and etiolated
leaves. It is easy to understand the immediate cause of the
normal formation of the cavity when observing that the
etiolated leaves have in cross-section a semilunar shape, with
peripheral cells slightly oblong but not of the well-known
H-palisade form, while the inflation of the leaf goes hand in
hand with the rapid growth and consequent tangential enlarge-
ment of these peripheral cells. In this case then, when we
prevent this inflation of the leaf by growing it in the dark, we
prolong the life of the central parenchymatous cells. Many
individuals have been thus grown in the dark, and leaves
a foot long produced altogether of living cells, whereas
normally the leaf becomes hollow at a distance of five to eight
centimetres from the point of its emergence from the bulb.

If zones of these leaves are etiolated by opaque wrappings
while the rest of the leaf is exposed to the light, the leaf will

' Sachs, Ueber den Einfluss det Tageslichtes auf NeubilduDg und Entfaltung
verschiedener Pflanzenorgane. Bot. Zeitung, 1863, BeiUge.

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412 Newcombe. — On the Cause and Conditions

become hollow through the etiolated zone, because there is in
such segments an expansion great enough to tear the central
cells. This expansion is to be traced to the effect of the
expansion in the adjacent inflated parts of the leaf. If how-
ever, instead of a yielding opaque band, gypsum ^ is used to
enclose zones of the leaf, the whole tissue of the enclosed part
will remain alive, though the cavity will exist as normally
both above and below the limits of the cast. By this process
the central mass of cells has been kept alive within the cast
eleven or twelve days after it had died outside of the limits
of the cast.

Similar results have been obtained by enclosing the aerial
shoots of Juncus effusus in gypsum. For eleven weeks some of
these young shoots were wholly encased in gypsum, at the
end of which period they showed the peripheral zone of living
cells thicker radially by two rows of cells than in normal shoots.
In this case, cells that weeks before would have passed over
into the dead, stellate form had, so far as cause can be dis-
cerned, been kept alive because they had not been subjected
to the usual stretching from the normal growth of the stem *.

The formation of the intercarinal canals in Equisetum
limosum was delayed for a week or more by the application
of a gypsum-cast to the base of a young shoot. The stem
did not, outside the cast, increase subsequently in diameter ;
hence the cast prevented only longitudinal extension of the
enclosed internodes. In Zea Mais the formation of the
lysigenous canal in the vascular bundles was prevented in

' The method of applying these casts is described by Pfeffer, in Berichte
d. K. Sachs. Gesellsch. d. Wissenschaften, December, 1892. The author, who
learned the method from PfefTer, has described it in Botanical Gazette, April,
1894, in an article entitled The Effect of Mechanical Resistance on the Develop-
ment and Life-period of Cells.

' It is probable that another factor comes into play in this case ; that is, that the
cells live longer, not only because they are not stretched by adjacent tissue, but
because they are prevented from making their own active and normal growth.
This question has been discussed by the author in The Effect of Mechanical
Resistance on the Growth of Plant-Tissues, Leipzig, 1893, and in The Effect of
Mechanical Resistance on the Development and Life-period of Cells, Botanical
Gazette, April and May, 1894.



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of Lysigefious Caviiy-formation. 4 1 3

like manner. The stem here did not increase in diameter
after the cast was applied, but the enclosed internode, when
examined, was but one-third the length of adjacent intemodes.
The small cells that are usually destroyed when the canal
is formed were living and intact about the annular vessel.

The explanation of the prolongation of the life-period of the
tissue in the forgoing five cases otherwise than as the result
of relieving the tension, seems to me improbable. In the one
case only of the leaves of Allium Cepa in gypsum-casts was
there any appreciable constriction of the part in which the
cells in question were preserved. When the constriction is
considerable, it is conceivable that the cells might by regu-
latory means be kept alive for purposes of transport But
I have produced in several species of plants, by means of
gypsum-casts, segments of stems with one-fourth the area in
cross-section of the same stem above and below the cast, and
all without an apparent effect on the transpiration or vitality
of the plants. Such plants have been obliged, therefore, to
carry their transpiration-current through a channel one-fourth
as great as in other parts of the stem. It is thus demonstrated
that, for ordinary transpiration, these plants do not need their
full amount of xylem and conducting tissue. Hence it is
pretty certain that the central cells in the segments of the
Onion- leaves encased in gypsum were not kept alive for needs
of transport, sinc6 the conducting channel in them was not
greatly narrowed by the cast.

Finally, the cases of Urtica dioica and Dahlia variabilis may
be again cited as furnishing evidence for the varying duration
of the life of the pith according to the tearing action of the
peripheral zones upon it. It will be remembered that in these
two species the cavity appears in thick stems during primary
growth, but in slender stems during secondary growth.

The life-period of cells cannot, however, be indefinitely
prolonged by averting destruction due to tension : in fact, the
destruction of cells by tension acting between tissues is an
indication that such cells are near the end of their life-period.
As already stated, etiolated leaves from the bulb of Allium



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414 Newtombe. — On the Cause and Conditions

Cepa preserve all their cells alive for a considerable period
after normal leaves become hollow. But it is also true that
in such etiolated leaves the central parenchyma finally dies
before the leaves die, and dies without collapsing. Again, the
intercarinal canals in Equisetum limosumy though their forma-
tion has been delayed for a time by gypsum-casts which
prevented the young intemodes from elongating, finally
appear within the limits of the casts as well as outside. In
Caltka palustris^ in whose stem a large lysigenous cavity
appears during primary growth, the death of the pith is
delayed by the use of gypsum-casts, but not altc^ether
prevented, unless the cast is put about the stem when the
latter is so young that the great prolongation of the life of the
pith must, as will be shown later, be looked upon as a regulatory
process. In the stems of this plant the tension is so much
reduced by the use of the casts that the pith does not collapse
when it dies, and we have the same condition as normally
occurs in Sambucus and other plants.

In Lamium garganicuntj Myrrhis odorata^ Archangelica
sativa and Melianthus major^ precisely similar results have
been obtained as with Caltka palustris. All of these plants
normally form a lysigenous cavity in the pith during primary
growth : all of them had casts applied about their stems
before any pith-cells died, yet not so early as to prevent the
pith-cells from attaining or nearly attaining their full size.
In all such cases the life of the pith was prolonged from one
to several weeks, but the pith then died without collapsing.

It is, of course, to be understood that to obtain these
results, and to be able to make the statements given, very
numerous experiments have been performed. Some of these
experiments under somewhat different conditions have given
results which are not stated in this place because they will
best be considered later under another heading. The one fact
which now demands special attention is the result that the
pith died in all these plants but a short time later than
normally and without being torn as it is usually; and this
leads to the conclusion that such cells would not live much



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of Lysigenous Cavity-formation. 415

longer than usually, were the normal tearing averted. In
other words, the tearing shortens only by a few days or weeks
what would otherwise be the life-period of the cells.



5. Effect of Prevention of the Extension of
Surrounding Tissues.

As shown on page 406, it is possible for a cavity to begin
schizogenously in secondary growth by the tension called
forth in the contraction of cells or tissues due to loss of turgor
while the surrounding tissues maintain their fixed position
and size. It is also conceivable that if, in stems in which such
a relation exists, the full primary extension of the vascular
zone and the cortex be prevented by the early application of
a gypsum-cast, there will arise less tension between pith and
v^cular zone when the former loses its turgfidity during
secondary growth. If the tension be thus averted it is quite
possible that the pith-cells would live longer, since they



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