Anatomy of Macadamia Seedlings

Anatomy of Macadamia Seedlings

 

John L. Price and Lois E. James*

Reprint from CMS 1967

The Macadamia tree is an unusual and interesting plant in many respects. Its history is colorful, involving early discovery by the Australian aborigines. Besides being a popular ornamental plant, the nut of the Macadamia is a relatively new hut exciting commercial crop, because of its distinctive taste, high yield, and slowness in turning rancid. The taxonomy of Macadamia also is interesting, in that it is a member of one of the more primitive plant families, the Proteaccae.

 

The microscopic anatomy of Macadamia is no less interesting, and there are many features of scientific and esthetic value. Since, with one exception (Metcalfe and Chalk, 1960), there is no anatomical description of Macadamia in the literature, this paper is given to acquaint persons interested in this plant with some of its anatomical features.

 

MATERIALS AND METHODS

 

Two greenhouse-grown seedlings (one of Macadamia tetraphylla and one of M. integrifolia, each approximately 18 inches high) were used in this study. Blocks of fresh tissue were excised from the plants and placed in a cryostat (freezing microtome), where they were quick-frozen and sectioned. The sections were then picked up on clean slides, air-dried in the cryostat chamber for 20 minutes, stained with crystal violet (Johansen, 1940) for 15 minutes, and mounted in balsam.

 

OBSERVATIONS

 

At the light-microscopic level, there are very few differences between the two seedlings; thus, the following general description applies to both species.

 

A typical cross-sectional view of a leaf is seen in fig. I., in which is shown the midrib and part of the leaf blade. One immediately notices the presence of many, dark-staining, thick-walled fiber cells (sclerophyllous tissue), which are common throughout most of the seedling tissues. They are generally considered to have a supportive function in plants. In the case of the leaf, this sclerophyllous tissue forms a sheath around the vascular bundles of the midrib and the larger veins have sheath extensions of sclerophyllous extensions, which connect to both epidermal layers. It is primarily the abundance of the thick-walled cells, which imparts a leathery texture to the leaves and forms thorns on their margins. The thin, black line, covering the leaf surfaces is a layer of wax and cutin, which retards water loss from the leaves and also contributes to their toughness. The lighter staining areas in the leaf are mesophyll tissues, which contain numerous chloroplasts (not visible). The thickness of the palisade layer varies between 20 and 40 percent of the leaf thickness (Duncan. 1960). Duncan (1960) reports also that the tissue arrangement varies among the leaves according to the amount of light they receive. For example, the palisade layer of a known shade leaf was found to constitute 22 percent of the leaf thickness, while the palisade layer of sun leaves constitute a greater percentage of the leaf thickness. The spongy layer of the mesophyll consists of loosely arranged cells.

 

The petiole of a M. integrifolia leaf is seen in. fig. 2. Individual fibers in the bundle can be seen easily. Such a view is not seen In M. tetraphylla, of course, which has sessile leaves. Note that there is no cuticle coating on this portion of the leaf.

 

Typical cross -sectionnal and longitudinal views of a stem 5-mm in diameter are seen in figs. 3 and 4 respectively. Even in such small stems, there is a large amount of woody tissue, which is typical of this plant family (Metcalfe and Chalk, 1950). This abundance of woody tissue gives an oak-like texture and pattern to the branches. There is also a relatively thick zone of cork tissue surrounding the stems, in which lenticels (air passages) may be seen. Stems of M. tetraphylla possess more vascular bundles than M. integrifolia, possibly a consequence of having more leaves per node (an average of 4 for the former and 3 for the latter).

 

The root anatomy is similar to that of the stem. As shown in fig. 5, the root is surrounded by a hand of cork tissue and contains many vascular bundles, which are usually smaller than those in the stem. The roots have a greater central pith area, in which many, small granules of storage starch can be seen.

 

Various tissues of Macadamia contain stone cells (sclereids), which is also a sclerophyllous tissue. These small, rounded cells have extremely thick walls, and are thought to have a supportive function, like the fiber cells. Examples of stone cells are shown in fig. 6, which is a cross-section of a stem node. The projection on the top is a portion of a leaf petiole, at the point where it attaches to the stem node. The stone cells are seen around this attachment zone and probably offer extra strength and support to the leaves. They are observed commonly also in the cortex region of stems, between the cork and vascular bundles.

 

1.Macadamia tetraphylla leaf cross-section. x300.

2.M. integrifolia petiole cross-section. x90.

3.M. integrifolia stem cross-section. x90.

4.M. integrifolia stem longitudinal section. x90.

5.M. tetraphylla root cross-section x90.

6.M. tetraphylla stem-node cross-section. x90.

 

 

 

(Abbreviations in figures: BS---bundle sheath: C-cork tissue; F---fiber cells: L---lentical: Md---midrib; MS---mesophyll; P---pith tissue; S---stone cells: V---vascular bundle.)

SUMMARY AND CONCLUSION

The most striking microscopic feature of Macadamia seedlings is the general abundance of sclerophyllous tissues, which impart a leathery texture to the leaves and considerable hardness to the stems and roots. There was a considerably larger amount of pitch tissue in M.tetraphylla, both in root and stem, as well as a greater number of vascular bundles in the stem. The leaves are almost identical microscopically.

In conclusion, the seedlings of Macadamia are structurally very strong, which is undoubtedly an important factor in their survival. Not only can they withstand strong winds and physical blows, but also their general hardness and thorny leaves offer much consternation to animals (people included!) who try to handle them.

 

*John L. Price is Ph.D. candidate in Plant Sciences Division, University of California, Riverside; Lois F. James is Chairman, Biology Department, Whittier College, Whittier, California and Vice-President, California Macadamia Society.

 

REFERENCES

 

I.Duncan, C. D. Comparative Morphological and Anatomical Investigations in the Commercial Species of the Genus Macadamia. (Unpublished thesis, Whittier College)

2.Pohansen, D. A. Plant Microtechnique. McGraw-Hill Book Co. 1940.

3.Metcalfe, CR. and L. Chalk. Anatomy of the Dicotyledons. Oxford Press, London. 1950.