Internal anatomy

The leaf anatomy of aloes was first studied for taxonomic purposes (Cutler, 1969). Later, when it became clear that aloe exudates could be of medicinal interest, anatomy was used to try and locate the cells or tissues in which particular substances arose or were stored. (Beaumont etal, 1985, 1986).

The internal anatomy of aloe leaves is fairly constant, regardless of the species. This means that apart from defining about three groups of miscellaneous species, transverse sections provide little information of wider systematic significance. Thicker leaves have more parenchymatous ground tissue, but the outer chlorenchyma layers, containing the green chloroplasts, and the flattened ring of vascular bundles just to the inner side of the chlorenchyma, are common to all. Initially, interest was focussed on the features that indicated adaptation to particular habitats. For example, thicker-leaved aloe species tend to be found in the harsher environments, where water supply is limited (e.g. Aloe dichotoma Masson, A. pegleri Schonl.). Thinner leaves occur in species from more moist habitats, for example A. haemanthifolia A.Berger et Marloth. Figure 16.1 shows a cross section of a typical leaf of A. kedongensis Reynolds.

Transverse Section Aloe Vera
Figure 16.1 Aloe kedongensis: transverse section of part of leaf. Arrow head shows thin walled cells of bundle cap. Magnification X 20.

Of particular interest is the cap of thin-walled, wide cells to the outer side of each vascular bundle. These strands contain exudates that may be coloured, and which are discussed in Chapter 3. The mucilage, which is of medicinal and cosmetic interest, is mainly found in the central cells of the parenchyma. Its function in the plant may be to store water. Beaumont etal. (1985, 1986) looked at the origins of the substances present in the bundle cap cells at the phloem poles. In the plant during life, it seems they are colourless, but on exposure to air, they often become coloured. The particular colour, purple, yellow or brown, for example, is typical of the species in which it is found. Granular particles had been observed in the bundle cap cells, and a suggestion is that these may represent precursors to the exudates. However, it still remains to be determined whether the exudates are synthesised in the cap cells themselves, or in other, surrounding cells. The exudates may dry, sealing a cut or damaged leaf. Aloe leaves are generally not damaged by insects or herbivores, so the exudates may also have properties that could deter browsing or insect attack.

Anatomy of leaf surfaces

Initial studies of aloe leaf surfaces with the light microscope indicated a possible rich source of diagnostic characteristics. There appeared to be a range of types of very fine surface sculpturing. This was difficult to see, because the very thick cuticle covering the epidermis had a rough interface with the outer wall of the epidermal cells. The interface appeared granular, and interfered with the interpretation of the fine surface features. However, the scanning electron microscope (SEM) proved ideal for such surface studies, and the beautiful but complex nature of these surfaces became readily understood. Since the electron beam does not penetrate deeply, a clear image of the surface itself could be produced. In addition to the sculpturing of the cuticle itself, the varied nature of epicuticular wax could also be determined (Cutler, 1979). The surface sculpturing was smooth, or composed of fine striations arranged in various ways, or of micropapillae. The size and frequency of the micropapillae are often diagnostic. These sculpturings are superimposed on the general form of the outer wall of the epidermal cells. These walls can be level, domed to varying degrees, or slightly sunken. When they are sunken, the outer edges of the side (anticlinal) walls of the epidermal cells may appear raised. A new terminology had to be developed.

The discovery of this rich source of information of potential taxonomic use was followed by a set of rigorous experiments involving anatomical and cytological studies, and crossing of species. Observations from field collected specimens of A. ferox Miller and A. africana Miller, which have a very wide distribution, showed little variation in their leaf surface sculpturing (Figure 16.3). Experiments have shown that this epidermal sculpturing is under close genetic control (e.g. Brandham and Cutler, 1978; Cutler etal, 1980; Carter etal., 1984). In crossing experiments between diploid and tetraploid species, it was possible to demonstrate the arms of particular chromosomes having coding for the normal sculpturing characters. Further experiments crossing Aloe with Gasteria and Haworthia species (Cutler, 1972; Cutler and Brandham, 1977) confirmed the strong genetic control of the characteristic sculpturing features. Cuticular sculpturing can therefore be used as a diagnostic tool, helping in the identification of species, or groups of similar species (Cutler, 1985). Two factors come into play. The first of these is relatedness. Closely related species tend to share similar epidermal features. Second, there is some genetic adaptation to environment. Normally this is expressed by surface roughness increasing in line with the ability of the species to withstand dryer and dryer habitats. Both factors influence the final appearance, and sometimes it may be that environmental adaptation overrides characters of relatedness (Cutler DF, 1982). Even so, looking at leaf surfaces under a high power epi-illuminating light microscope, or preferably an SEM, it is often possible to say which Aloe species are closely related to one another.

Stomata are usually deeply sunken in species that normally grow in exposed, water-stressed conditions. The stomata are overarched by four well developed lobes, one from each of the four surrounding epidermal cells. Below these is an elliptical, dome like structure, with an axial slit in it. This is formed of cuticle. Only when we look deeper into the stoma do we see the guard cells themselves, and the aperture that can be opened or closed, according to prevailing conditions.

It is easy to detect those species that grow in areas that are generally more moist, such as A. haemanthifolia, a species that grows among grasses and other herbaceous vegetation, and A. ciliaris Haw., a species that scrambles through shrubs and trees on river banks, since their stomata are not sunken and have no overarching lobes (Cutler, 1982).

The results of these studies confirmed that in aloe leaves the surface sculpturing could be used safely to:

• help identify sterile plants, or leaf fragments;

• indicate relationship status of given species;

• provide some information on habitat 'preference'.

The plates illustrate selections from the range of surface features. Some show the similarity between closely related species, and others striking adaptation to habitat preference. Note that the cuticular sculpturing is sometimes obscured by surface wax. This can be removed chemically to expose the sculpturing beneath, but it too is of diagnostic significance and can provide further clues to the identity of an unknown plant lacking flowers.

Aloe Vera Plant Anatomy
Figure 16.2 Aloe gracilis: leaf surface showing superficial stomata without pronounced surrounding lobes. Anticlinal walls are indicated by shallow grooves; sculpturing consists of striations variously jointed into a loose reticulum. Magnification X 300.
Aloe Waxy Surface

Figure 16.3 Aloe africana: leaf surface with raised ridges above anticlinal walls and small micropapillae.

The stoma is deeply sunken and the four surrounding lobes almost occlude the cavity above the guard cells. Magnification X 300.

Figure 16.3 Aloe africana: leaf surface with raised ridges above anticlinal walls and small micropapillae.

The stoma is deeply sunken and the four surrounding lobes almost occlude the cavity above the guard cells. Magnification X 300.

Aloe Africana

Figure 16.4 Aloe pegleri: leaf surface with copious wax particles partly obscuring the surface sculpturing.

Each epidermal cell has a prominent central papilla and the lobes surrounding a sunken stoma are well developed. Magnification X 300.

Figure 16.4 Aloe pegleri: leaf surface with copious wax particles partly obscuring the surface sculpturing.

Each epidermal cell has a prominent central papilla and the lobes surrounding a sunken stoma are well developed. Magnification X 300.

Leaf Surface Wax
Figure 16.5 Aloe mutabilis: leaf surface, each epidermal cell has a domed outer wall with smaller micropapillae surrounding larger central ones. Lobes surrounding the sunken stomata are well developed. Fine wax particles cover the surface. Magnification X 300.
Sunken Well
Figure 16.6 Aloe arborescens: leaf surface with fine micropapillae covering the flat outer walls of the epidermal cells. As in A. mutabilis, to which it is thought to be closely related, the lobes surrounding the sunken stoma are well developed. Magnification X 300.

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Aloe and Your Health

Aloe and Your Health

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