In the last number of this journal it was shown how plants seek to
avoid the visits of unsuitable insects to their flowers. This is one
means of protection, but there are many others which are even more
striking and vital. It is supposed by many that plants are helpless
beings, which must submit to all sorts of unfavorable conditions which
come upon them. This is far from true, for while plants as a rule are
fixed and unable to escape from danger by flight, still they have very
many ways of helping themselves.
Prominent among the dangers which come to active green plants are
those which arise from too intense light, which may destroy the
delicate working substances. Since the leaves are the great working
organs in the manufacture of food, they are especially equipped for
protection. Those leaves which must work in exposed places have many
details of structure which are evidently for guarding them against
the ill effects of too intense light. The most striking adaptations,
however, are those which have to do with protective positions. Under
ordinary circumstances leaves are placed so that their flat faces are
exposed to the most intense light. In some cases this is so great a
danger that the leaves are set edgewise, the edges being directed
upwards and downwards. When a plant assumes this habit, the leaves are
said to be in a profile position, and the plants are sometimes called
"compass plants." The latter name has come from the fact that such
leaves usually point north or south, and once it was assumed that this
position was in response to some mysterious magnetic influence. It is
found, however, that it is merely an effort on the part of the plant
to protect its leaves from the intense light of midday, and at the
same time to expose them to the morning and evening rays of much less
intensity. If a leaf is to be placed with its edge upwards and its flat
faces east and west, it follows of necessity that it will point either
north or south.
Some leaves, however, have the power of shifting their position
according to their needs, directing their flat surfaces toward the
light, or more or less inclining them according to the danger.
Perhaps the most completely adapted leaves of this kind are those
of the "sensitive plants," whose leaves respond to various external
influences by changing their positions. The sensitive plants abound
in dry and hot regions, and one of the best known is represented in
our illustration. It will be noticed that the leaves of this Mimosa
are divided into very numerous small leaflets, which stretch in pairs
along the leaf branches. When the time of intense light and dryness
approaches some of the pairs of leaflets fold together, slightly
reducing the surface exposure. As the unfavorable condition continues,
more leaflets fold together, then still others, until finally all the
leaflets may be folded together, and the leaves themselves may bend
against the stem. It is like a sailing vessel gradually taking in sail
as a storm approaches, until finally nothing is exposed, and the vessel
weathers the storm by presenting only bare poles. These are but a few
illustrations of the very numerous devices for escaping too intense
light and the dangers which accompany it.
[Illustration: SENSITIVE PLANT.
Awake. (Mimosa pudica.) Asleep.
PLANTS PRESENTED BY LINCOLN PARK COMMISSIONERS.
COPYRIGHT 1900, BY
A. W. MUMFORD, CHICAGO.]
One common danger in temperate regions comes from the lowering of the
temperature each night, which sometimes may chill the living substances
to the danger point. This is particularly dangerous to seedlings, whose
tender structures have not yet developed the ordinary protective
coats. In the spring the seed leaves of numerous seedlings may be seen
at the approach of night to rise upward and come together, just as the
palms of the hand may be placed together over one's head. This reduces
the surface of exposure and the danger of chill at least one-half.
Darwin experimented upon these seedlings, and discovered that by
preventing some of the seed leaves from moving, the seedlings were
seriously injured. The leaves of very many plants assume a peculiar
night position which tends to meet the danger of loss of heat. Often
the three leaflets of the common clover, if growing in an exposed
place, may be observed to fold together into a sort of tent-like
arrangement.
Many plants are also observed to protect themselves against rain, as
it is necessary for leaves to avoid becoming wet. If the water is
allowed to soak in, the work of the leaves is at once interfered with.
Hence it will be noticed that most leaves are able to shed water,
partly by their position, partly by their structure. In many plants
the leaves are so arranged that the water runs off toward the stem; in
other plants the rain is shed outwards as from the eaves of a house.
Some of the structures which prevent the rain from soaking in are a
smooth epidermis, layers of cuticle, hairy coverings, etc. Interesting
experiments may be performed with different leaves to test their power
of shedding water. If a gentle spray be allowed to play upon different
plants it will be observed that the water glances off at once from the
surfaces of some leaves, runs off more slightly from others, and may be
more or less retained by others.
Perhaps the most general preparation for protection in our region is
that which is made for the coming of the winter's cold. In many cases
plants do not attempt to protect their delicate structures from the
severity of winter, but disappear entirely, leaving only well-protected
seeds to carry them over into the next growing season. This results in
the so-called "annual habit," which has been learned by many plants
in order to escape a season of danger. Other plants do not disappear
so completely, but everything above the surface of the ground dies,
while the plant continues in the form of underground bulbs, tubers,
or various thickened structures. This habit of seeking a subterranean
retreat at the approach of some dangerous season is a very good one,
and is found in many of our early spring plants. This subterranean
habit has a great advantage over the annual habit, since a seed is very
slow in bringing the plant back again, while a bulb can produce its
plant very rapidly.
Still other plants preserve more of their structures than either the
annuals or the ground-loving plants. For example, most of our trees
have cultivated what is known as the deciduous habit, that is, they
merely drop their leaves, which are the endangered structures, at the
approach of the unfavorable season, and renew them again when the
favorable conditions return. It should be remarked that these leaves
do not fall because they are broken off, but that in a certain sense
it is a process of growing off, which is carefully prepared for. One
of the most prominent features associated with the deciduous habit is
the autumnal coloration. The vivid colors which appear in the leaves of
many trees just before the time of falling have attracted a great deal
of attention, but although it is so prominent, the causes for it are
very obscure. It will be noticed that this autumnal coloration consists
in the development of various shades of two typical colors, yellow and
red. It is known that the yellow is due to the breaking down of the
green substances, so that it simply indicates a post mortem change, as
may be noticed in connection with the blanching of celery in which the
leaves and upper part of the stem may be green, the green may shade
gradually into yellow, and finally into the pure white of complete
blanching. The red coloring matter, however, is very different. Certain
experiments upon plant colors have indicated that the presence of the
red slightly increases the temperature by absorbing more heat. It is
suggested that the red color may be a slight protection to the living
substance which is ceasing to work, and which is in danger of exposure
to cold. If this be true, it may be that the same explanation will
cover the case of the red flush so conspicuous in buds and young leaves
in early spring. It must not be supposed that the need of protection
has developed the coloring, but since it is developed it may be of some
such service to the plant. Even the conditions which determine autumnal
coloration have not been made out certainly.
It is instructive to notice how differently the so-called evergreens,
as pines, spruces, etc., have answered the problem of protection
against the cold of winter. The evergreens, instead of dropping their
leaves, have undertaken to protect them, giving them a small surface
and very heavy walls. In this way protection has been secured at the
expense of working power during the season of work. Reduced surface and
thick walls are both obstacles to leaf work. On the other hand, the
deciduous trees have developed the working power of their leaves to the
greatest extent, giving them large surface exposure and comparatively
delicate walls. It is out of the question to protect such an amount
of surface during the winter, and hence the deciduous habit. The
evergreens are saved the annual renewal of leaves, but lose in working
power; the deciduous trees must renew their leaves annually, but gain
greatly in working power.
To obtain the most striking instances of protection, however, one must
examine plants which belong to permanently dry regions, such as may be
found in the United States along the Mexican border, or in the regions
of tropical deserts. In the first place, it will be noticed that the
plants in general produce smaller leaves than in other regions. That
this holds a direct relation to the dry conditions is evident from
the fact that the same plant often produces smaller leaves in dry
conditions than in moist. One of the most striking features of an arid
country is the absence of large leaves. These reduced leaves are of
various forms, such as the needle leaves of pines, or the thread-like
leaves of certain sedges and grasses, or the narrow leaves with
inrolled margins such as is common in many heath plants. The extreme of
leaf reduction has been reached by the Cactus plants, whose leaves, so
far as foliage is concerned, have disappeared entirely, and the leaf
work is done by the surface of the globular, cylindrical, or flattened
stems. A covering of hairs is an effective sun screen, and it is very
common to find plants of dry regions characteristically hairy. In
such regions it is to be observed also that dwarf growths prevail, so
that the plant, as a whole, does not present such an exposure to the
drouth as in regions of greater moisture. One of the most prominent
measures of protection in dry regions is the organization of what are
known as water reservoirs. Nearly all plants of such regions have
leaves which are known as fleshy, that is, they are thick and juicy,
being reservoirs of stored up moisture which is doled out cautiously
according to the needs of the plant, without any wastefulness.
The whole subject of plant protection is an immense one, and the
illustrations given above are merely intended to suggest that there is
such a subject, and to lead to some observation of the various schemes
of protection which are to be seen plainly on every hand.
John Merle Coulter.
* * * * *
Nature is but a name for an effect
Whose cause is God.
--Cowper, "The Task."