A dictionary of modern gardening - University Library

HE A 282 HE A
instances, are so complex]y combined growth of the plant to diminish and its
that the plant has not sufficient power
to force them open to permit the protrusion
of the seed-stem. The close-
colour to become more pale ; this effect
being now produced by the plant's torpidity,
or want of excitement to perform
ness of the heading is regulated by the
exposure to the light. In a shady situ-
the requisite elaboration of the sap, as
it is by over-excitement when made to
ation all the leaves are required to ela- vegetate in a temperature which is too
borate the sap, on account of the defi- elevated.
cient light rendering each less active; If blossoms are produced at all, they
therefore they open as they are formed. are unfertile, and the entire aspect o
In a free exposure a few leaves are able the plant betrays that its secretions are
to effect the requisite decomposition ;
and hence the reason why cabbages always
have " harder hearts'''' in summer
not healthy and its functions are deadened.
Mr. Knight says, "that melon
and cucumber plants, if grown in a
than in spring or autumn, when the temperature too low, produce an excess
light is less intense.
HEADING-DOWN is cutting off en-
'tirely or to a considerable extent, the
branches of a tree or shrub—a process
of female blossoms; but if the temperature
be too high, blossoms of the opposite
sex are by far too profuse." The
drier the air the greater is the amount
not rashly to be resorted to, and adapted of moisture transpired ; and this be-
only to reduce them when the plant comes so excessive, if it be also pro-
seems declining in vigour, or has attainmoted by a high temperature, that
ed an undesirable size.
HEART'S-EASE. See Pansy.
HEAT is the prime agent employed
plants in hot-houses, where it has occurred
often, dry up as if burned. The
justly lamented Mr. Daniell has well
by the Almighty Creator to call vege- illustrated this by showing, that if the
table life into existence, to develop temperature of a hot-house be raised
vegetable form, to effect all vegetable
changes, and to ripen all vegetable
only five degrees, viz. from
produce. All these effects are performed
most efficiently, in the case of
every plant, at some different tempera
75'' \
,
^
'
to 80",
whilst the air within it retains the same
degree of moisture, a plant that in the
lower temperature exhaled fifty-seven
grains of moisture, would in the higher
ture or degree of heat; and he who temperature, exhale one hundred and
ascertains most correctly those heats, twenty grains in the same space of
has taken a gigantic step towards excellence
as a gardener. An uncongenial
time.
Plants, however, like animals, can
heat is as pernicious to vegetables as to bear a higher temperature in dry air
animals. Every plant has a particular than they can in air charged with vatemperature
without which its functions pour. Animals are scalded in the latcease
; but the majority of them luxuri- ter if the temperature is very elevated,
ate most in a climate of which the and plants die, under similar circumextreme
temperature does not much stances, as if boiled. MM. Edwards
exceed 32° and 90°. No seed will and Colin found kidney-beans sustained
vegetate—no sap will circulate—at a no injury, when the air was dry, at a
temperature at or below the freezing
point of water. No cultivation will
temperature of IG?^; but they died in
a few minutes if the air was moist.
renderplants, natives of the torrid zone Other plants under similar circumcapable
of bearing the rigours of our stances, would perish probably at a
winters, although their offspring, raised much lower temperature ; and the fact
from seed, may be rendered much more affords a warning to the gardener to
hardy than their parents. Others are have the atmosphere in his stoves very
capable of resisting the greatest known dry whenever he wishes to elevate their
!
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cold to which they can be exposed ; yet temperature for the destruction of inall
have degrees of temperature most sects or other purposes,
congenial to them, and if subjected to Some plants, like some animals, are
lower temperatures, are less or more able to endure a very high degree ofteminjured
proportionately to the intensity perature. Sir Joseph Banks and others
of that reduction. If the reduction of have breathed for many minutes in an attemperature
be only slightly below that mosphere hot enough to cook eggs ; and
which is congenial, it only causes the I have myself travelled in Bengal breath-