CHAPTER XII

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.