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 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
colored, 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 wallflower; (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 vapors (_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
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. 116.--STOMATES OF GERANIUM LEAF.]
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 each other at their middles on either side of 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 vapor from passing out.
_When a leaf is actively at work making new organic compounds, the
stomates are usually open; when unfavorable 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 vapors 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.
[Illustration: FIG. 117.--LENTICELS ON YOUNG SHOOT OF RED OSIER
(CORNUS).]
=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 the plane surface? Color? 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, color, 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.