LEAVES—STRUCTURE OR ANATOMY
Besides the =framework=, or system of veins found in blades of all
leaves, there is a soft cellular tissue called =mesophyll=, or =leaf
parenchyma=, and an =epidermis= or skin that covers the entire
outside part.
[Illustration: FIG. 113.—SECTION OF A LEAF, showing the air-spaces.
Breathing-pore or stoma at _a_. The palisade cells which chiefly
contain the chlorophyll are at _b_. Epidermal cells at _c_.]
=Mesophyll=.—The mesophyll is _not all alike or homogeneous_. The
upper layer is composed of elongated cells placed perpendicular to
the surface of the leaf. These are called =palisade cells=. These
cells are usually filled with green bodies called =chlorophyll
grains=. The grain contains a great number of chlorophyll drops
imbedded in the protoplasm. Below the palisade cells is the spongy
=parenchyma=, composed of cells more or less spherical in shape,
irregularly arranged, and provided with many intercellular air
cavities (Fig. 113). In leaves of some plants exposed to strong
light there may be more than one layer of palisade cells, as in the
India-rubber plant and the oleander. Ivy when grown in bright light
will develop two such layers of cells, but in shaded places it may
be found with only one. Such plants as iris and compass plant, which
have both surfaces of the leaf equally exposed to sunlight, usually
have a palisade layer beneath each epidermis.
=Epidermis.=—The outer or epidermal cells of leaves do not bear
chlorophyll, but are usually so transparent that the green mesophyll
can be seen through them. They often become very thick-walled, and
are in most plants devoid of all protoplasm except a thin layer
lining the walls, the cavities being filled with cell sap. This sap
is sometimes coloured, as in the under epidermis of begonia leaves.
It is not common to find more than one layer of epidermal cells
forming each surface of a leaf. The epidermis _serves to retain
moisture_ in the leaf and as a general _protective covering_. In
desert plants the epidermis, as a rule, is very thick and has a dense
cuticle, thereby preventing loss of water.
There are various _outgrowths of the epidermis_. =Hairs= are the
chief of these. They may be (1) =simple=, as on primula, geranium,
nægelia; (2) =once branched=, as on wall-flower; (3) =compound=,
as on verbascum or mullein; (4) =disk-like=, as on shepherdia; (5)
=stellate=, or star-shaped, as in certain crucifers. In some cases
the hairs are =glandular=, as in Chinese primrose of the greenhouses
(_Primula Sinensis_) and certain hairs of pumpkin flowers. The hairs
often protect the breathing-pores, or stomates, from dust and water.
=Stomates= (sometimes called =breathing-pores=) _are small openings
or pores_ in the epidermis of leaves and soft stems that allow the
passage of air and other gases and vapours (_stomate_ or _stoma_,
singular; _stomates_ or _stomata_, plural). They are _placed near the
large intercellular spaces_ of the mesophyll, usually in positions
least affected by direct sunlight. Fig. 114 shows the structure.
There are =two guard-cells= at the mouth of each stomate, which may
in most cases open or close the passage as the conditions of the
atmosphere may require. The guard-cells contain chlorophyll. In Fig.
115 is shown a case in which there are compound guard-cells, that
of ivy. On the margins of certain leaves, as of fuchsia, impatiens,
cabbage, are openings known as =water-pores=.
[Illustration: FIG. 114.—DIAGRAM OF STOMATE OF IRIS (Osterhout).]
[Illustration: FIG. 115.—STOMATE OF IVY, showing compound
guard-cells.]
_Stomates are very numerous_, as will be seen from the numbers
showing the pores to each square inch of leaf surface:
Lower surface Upper surface
Peony 13,790 None
Holly 63,600 None
Lilac 160,000 None
Mistletoe 200 200
Tradescantia 2,000 2,000
Garden Flag (iris) 11,572 11,572
[Illustration: FIG. 116.—STOMATES OF GERANIUM LEAF.]
The arrangement of stomates on the leaf _differs with each kind of
plant_. Fig. 116 shows stomates and also the outlines of contiguous
epidermal cells.
[Illustration: FIG. 117.—LENTICELS ON YOUNG SHOOT OF RED OSIER
(CORNUS).]
The function or work of the stomates is to _regulate the passage of
gases_ into and out of the plant. The directly active organs or parts
are guard-cells, on either side the opening. One method of opening is
as follows: The thicker walls of the guard-cells (Fig. 114) absorb
water from adjacent cells, these thick walls buckle or bend and part
from one another at their middles on either side the opening, causing
the stomate to open, when the air gases may be taken in and the leaf
gases may pass out. When moisture is reduced in the leaf tissue,
the guard-cells part with some of their contents, the thick walls
straighten, and the faces of the two opposite ones come together,
thus closing the stomate and preventing any water vapour from passing
out. _When a leaf is actively at work making new organic compounds,
the stomates are usually open; when unfavourable conditions arise,
they are usually closed._ They also commonly close at night, when
growth (or the utilizing of the new materials) is most likely to be
active. It is sometimes safer to fumigate greenhouses and window
gardens at night, for the noxious vapours are less likely to enter
the leaf. Dust may clog or cover the stomates. Rains benefit plants
by washing the leaves as well as by providing moisture to the roots.
=Lenticels=.—On the young woody twigs of many plants (marked in
osiers, cherry, birch) there are small corky spots or elevations
known as =lenticels= (Fig. 117). They mark the location of some
loose cork cells that function as stomates, for _green shoots_, as
well as leaves, take in and discharge gases; that is, soft green
twigs _function as leaves_. Under some of these twig stomates,
corky material may form and the opening is torn and enlarged: _the
lenticels are successors to the stomates_. The stomates lie in the
epidermis, but as the twig ages the epidermis perishes and the
bark becomes the external layer. _Gases continue to pass in and out
through the lenticels_, until the branch becomes heavily covered
with thick, corky bark. With the growth of the twig, the lenticel
scars enlarge lengthwise or crosswise or assume other shapes, often
becoming characteristic markings.
=Fibro-vascular Bundles.=—We have studied the fibro-vascular
bundles of stems (Chap. X). These stem bundles _continue into the
leaves, ramifying into the veins_, carrying the soil water inwards
and bringing, by diffusion, the elaborated food out through the
sieve-cells. Cut across a petiole and notice the hard spots or areas
in it; strip these parts lengthwise of the petiole. What are they?
=Fall of the Leaf.=—In most common deciduous plants, when the
season’s work for the leaf is ended, the nutritious matter may be
withdrawn, and _a layer of corky cells is completed over the surface
of the stem where the leaf is attached_. _The leaf soon falls._ It
often falls even before it is killed by frost. Deciduous leaves
begin to show the surface line of articulation in the early growing
season. This articulation may be observed at any time during the
summer. The area of the twig once covered by the petioles is called
the =leaf-scar= after the leaf has fallen. In Chap. XV are shown a
number of leaf-scars. In the plane tree (sycamore or buttonwood), the
leaf-scar is in the form of a ring surrounding the bud, for the bud
is covered by the hollowed end of the petiole; the leaf of sumac is
similar. Examine with a hand lens leaf-scars of several woody plants.
Note the number of bundle-scars in each leaf-scar. Sections may be
cut through a leaf-scar and examined with the microscope. Note the
character of cells that cover the leaf-scar surface.
SUGGESTIONS.—_To study epidermal hairs_: =75.= For this study, use
the leaves of any hairy or woolly plant. A good hand lens will
reveal the identity of many of the coarser hairs. A dissecting
microscope will show them still better. For the study of the cell
structure, a compound microscope is necessary. Cross-sections may
be made so as to bring hairs on the edge of the sections; or in
some cases the hairs may be peeled or scraped from the epidermis
and placed in water on a slide. Make sketches of the different
kinds of hairs. =76.= It is good practice for the pupil to describe
leaves in respect to their covering: Are they smooth on both
surfaces? Or hairy? Woolly? Thickly or thinly hairy? Hairs long or
short? Standing straight out or lying close to the surface of the
leaf? Simple or branched? Attached to the veins or to the plane
surface? Colour? Most abundant on young leaves or old? =77.= Place
a hairy or woolly leaf under water. Does the hairy surface appear
silvery? Why? _Other questions_: =78.= Why is it good practice to
wash the leaves of house plants? =79.= Describe the leaf-scars
on six kinds of plants: size, shape, colour, position with
reference to the bud, bundle-scars. =80.= Do you find leaf-scars on
monocotyledonous plants—corn, cereal grains, lilies, canna, banana,
palm, bamboo, green brier? =81.= Note the table on page =88.= Can
you suggest a reason why there are equal numbers of stomates on
both surfaces of leaves of tradescantia and flag, and none on upper
surface of other leaves? Suppose you pick a leaf of lilac (or some
larger leaf), seal the petiole with wax and then rub the under
surface with vaseline; on another leaf apply the vaseline to the
upper surface; which leaf withers first, and why? Make a similar
experiment with iris or blue flag. =82.= Why do leaves and shoots
of house plants turn towards the light? What happens when the
plants are turned around? =83.= Note position of leaves of beans,
clover, oxalis, alfalfa, locust, at night.