CHAPTER VI

LEAVES

Leaves collectively constitute the greatest manufacturing plant in the world. Most of the food, clothing, and medicine used by man is formed as a result of the work of the leaf. The cell contents, structure, and arrangement of the different cells of the leaf differ in a marked degree from the cell contents, structure, and arrangement of the cells in the other organs of the plant. This accounts for the presence of the large amount of chlorophyll in the leaf, the presence of stomata, and the peculiar arrangement of the cells.

It should be ascertained if the stomata are above, even with, or below the epidermis; the nature of the epidermal cells, and, when present, the nature of the hypodermal cells; the number of layers of palisade parenchyma and whether it is present on both surfaces of the leaf, and the nature of the outgrowths from the epidermal cells.

KLIP BUCHU

The cross-section of klip buchu (Plate 108) has the following structure:

=Epidermis.= The epidermal cells of klip buchu are modified to form papillæ, the walls are yellowish white, and the papillate portion of the cell is nearly solid.

=Hypodermis.= The hypodermal cells are never intact because the mucilage contained in the cells swells when placed in water and breaks the thin side walls.

=Upper Palisade Parenchyma.= The palisade parenchyma is two layers in thickness. The cells of the outer layer are greatly elongated and are packed with chlorophyll. The inner layer of palisade cells is more irregular, and the cells are much shorter than the cells of the outer palisade layer.

[Illustration: PLATE 108

CROSS-SECTION OF KLIP BUCHU JUST OVER THE VEIN

_A._ Papillate upper epidermis. _B._ Hypodermal cells with broken side walls, due to expansion of mucilage contents. _C._ Palisade cells, showing two cells filled with chlorophyll. _D._ Palisade like mesophyll. _E._ Endodermis. _F._ Vascular strand of vein. _G._ Conducting cells with spirally thickened walls. _H._ Characteristic leaf mesophyll. _I._ Short, thick palisade cells on the under side of leaf, just under the vein. _J._ Under hypodermal cells. _K._ Papillate under epidermis.]

=Spongy Parenchyma.= The spongy parenchyma cells are branched; therefore, large intercellular spaces occur between the cells.

=Under Palisade Parenchyma.= The palisade cells of the under epidermis are short and broad, and they contain fewer chlorophyll grains than the upper palisade cells of the upper epidermis. These cells occur only under the veins.

=Under Hypodermis.= The under hypodermal cells are shorter and broader than the upper hypodermal cells.

=Under Epidermis.= The under epidermal cells are modified to form papillæ which are similar to the papillæ of the upper epidermis.

=Fibro-Vascular Bundle.= The cells composing the vascular bundle are sieve cells, vessels, and fibres.

The =sieve cells= are small and the walls are white and angled.

The =vessels= have thick, white, angled walls.

The =bast fibres= are rounded in outline and the walls are thick and white.

=Endodermis.= The endodermal cells encircle the fibro-vascular bundles. The cells are large, thin-walled, and oval in shape.

=Secretion Cells.= Near the edges of the leaf are schizogenous secretion cavities surrounded by thin-walled secretion cells.

POWDERED KLIP BUCHU

When the leaf is powdered (Plate 109), the cells are quite as characteristic in appearance. The upper epidermal cells (1) have thick-beaded, yellowish-white walls and papillate outer walls. No stomata occur on the upper surface. The under epidermis (2) with numerous stomata, is surrounded by the characteristic guard cells. The end walls are beaded as on the upper surface. The palisade cells (3) appear as in the cross-section. The conducting cells (4 and 4) are of the spiral and pitted type. The papillæ (5 and 5) are very abundant in the powder and very characteristic. The fragments of the epidermis (6) are also abundant. The mesophyll (7) is characteristic, as it retains its form when powdered. The fibres (8) are usually associated with the conducting cells; occasionally they are found free as in the illustration.

[Illustration: PLATE 109

POWDERED KLIP BUCHU

1. Upper epidermis. 2. Under epidermis. 3. Palisade cells with chlorophyll. 4 and 4. Conducting cells. 5 and 5. Papillæ. 6. Fragments of the epidermis. 7. Mesophyll. 8. Fibres.]

MOUNTAIN LAUREL

=Epidermis.= The epidermal cells of mountain laurel are occasionally modified, as unicellular hairs (Plate 110, Fig. 1), particularly in the region of the veins. The ordinary epidermal cells have thick outer walls and thin inner walls. Beneath many of the epidermal cells are large air-spaces.

=Upper Palisade Parenchyma.= The palisade parenchyma vary from four to five layers. The inner palisade cells are shorter and broader than the outer layer of cells.

=Parenchyma.= The parenchyma cells (Fig. 4) are rounded in form and they are arranged in the form of columns which are one cell in thickness above, but two to three cells in thickness near the under epidermis. Between each chain of cells is a larger intercellular space (Fig. 6). In a few of the cells are large rosette crystals.

=Under Epidermis.= The under epidermal cells are uniformly smaller than the upper epidermal cells.

It is thus seen that mountain laurel leaf has no hypodermal cells; no spongy parenchyma; no under palisade cells; no under hypodermal cells, and no secretion cavities.

TRAILING ARBUTUS

=Epidermis.= The epidermal cells of the trailing arbutus (Plate 111, Fig. 2) are variable in size. Many of the cells are modified, as guard cells (Fig. 1).

=Parenchyma.= The parenchyma cells are round and they are compactly arranged (Fig. 3) on the upper side of the leaf, but on the under side they are arranged in round, small, intercellular spaces (Fig. 5). In some of the intercellular spaces are rosette crystals (Fig. 7).

=Under Epidermis.= The under epidermal cells are smaller than the upper epidermal cells.

It will be seen that the structure of trailing arbutus leaf is very simple and that its structure is different from that of klip buchu and mountain laurel.

The structure of powdered leaves is very variable, yet characteristic for a given species. The leaves from the insect flower plant are collected with the stems, and ground and sold as a substitute for insect flowers. These leaves, when powdered, show the following structure (Plate 112):

[Illustration: PLATE 110

CROSS-SECTION MOUNTAIN LAUREL (_Kalmia latifolia_, L.)

1. Hair. 2. Epidermis. 3. Palisade parenchyma. 4. Parenchyma. 5. Under epidermis. 6. Intercellular space. 7. Rosette crystal. 8. Chlorophyll.]

[Illustration: PLATE 111

CROSS-SECTION TRAILING ARBUTUS LEAF (_Epigæa repens_, L.)

1. Stomata. 2. Epidermis. 3. Parenchyma. 4. Cell with chlorophyll. 5. Intercellular space. 6. Under epidermis. 7. Rosette crystal.]

Both the upper and lower epidermis have stomata (Figs. 1 and 2), but they differ in that the surrounding cells of the upper epidermis are wavy, while the corresponding cells of the under epidermis are similar, though the under epidermis has many attached hairs (Figs. 3 and 4). The T-shaped hairs form the most abundant element of the powder. They are similar in structure to those found on the scales and stem. Fragments of the mesophyll have round cells and contain chlorophyll (Fig. 6). The conducting cells are spiral or reticulate.

The different cells of the leaf differ greatly in structure, in amount, and in arrangement. In uva-ursi, boldus, pilocarpus, eucalyptus, and chimaphila leaves the outer walls of the epidermal cell is very thick. In uva-ursi leaves this thick wall appears bluish green when viewed under low power of the microscope.

In belladonna, stramonium, henbane, peppermint, spearmint, digitalis, and horehound, the outer wall of the epidermal cells is thin.

In witch-hazel, stramonium, coca, phytolacca, and peppermint there is a single layer of palisade parenchyma on the upper surface only of the leaf.

In senna there is one layer of palisade parenchyma on the upper and one layer on the under side of the leaf. In matico and tea leaves there are two layers of spongy parenchyma on the upper side of the leaf.

In chestnut leaves there are three layers of palisade parenchyma on the upper side of the leaf.

In eucalyptus leaves the entire central part of the leaf, with the exception of the secretion cells and fibro-vascular bundle, is made up of the palisade parenchyma.

In some leaves no palisade parenchyma occurs. Trailing arbutus (Plate 111) is an example of such a leaf.

In stramonium leaves the spongy parenchyma is strongly branched; in mountain laurel the spongy parenchyma is mostly non-branched and circular in form, as in trailing arbutus (Plate 111, Fig. 3), and as occurs in the midrib portion of most leaves.

[Illustration: PLATE 112

POWDERED INSECT FLOWER LEAVES (_Chrysanthemum cinerariifolium_ [Trev.], Vis.)

1. Upper epidermis. 2. Under epidermis showing stoma and hair scar. 3. Cross-section of under epidermis with attached hair. 4. Cross-section of upper epidermis. 5. Hairs. 6. Mesophyll with chlorophyll bodies. 7. Conducting cells.]

In stramonium and chestnut are found rosette crystals. In henbane, coca, and senna are found prisms. In belladonna, scapola, and tobacco leaves are found micro-crystals. In most leaves no crystals occur. In witch-hazel and tea leaves stone cells occur, but in most leaves there are no stone cells. In eucalyptus, thyme, jaborandi, buchu, rosemary, and white pine leaves are secretion cells; while in belladonna, stramonium cells occur. In senna and coca leaves are crystal-bearing fibres; most leaves do not have crystal-bearing fibres.

In chimaphila and uva-ursi there are no outgrowths from the epidermal cells.

In senna, witch-hazel, chestnut, and coca, numerous non-glandular hairs occur on the epidermis. In tobacco, belladonna, henbane, pennyroyal, peppermint, and spearmint both glandular and non-glandular hairs occur on the epidermis.

When studying leaves there should be considered the absence or presence of outgrowths and their nature; the nature of the epidermis and, when present, the number of layers of the hypodermis; the nature of the stoma, whether raised above, even with, or below the level of the epidermis; the number of layers, and the distribution, when present, of the palisade parenchyma; the form and amount of the spongy parenchyma; the absence or presence of secretion cells; the nature and form of the fibro-vascular bundles, and the nature and amount of the organic and inorganic cell contents.

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