The Journal of Indian Botany/Volume 1/May 1920/Variegation in certain Cultivated Plants

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VARIEGATION IN CERTAIN CULTIVATED PLANTS

M. BALASUBRAMANYAM, B.A., (Hons.), Sacred Heart College, Shembaganur.

The interesting paper by Bateson on Variegation* in certain plants, led me to examine at the instance of Professor Fyson some of the variegate species grown in Madras gardens, for the histological details of the differently coloured patchos.

In Pothos aurea, Linden, the patches of green and yellowish - white parts are not sharply defined, partly because the loss of chlorophyll as one passes from a green to a light part is gradual, and partly because the green occurs in flecks in the light parts, and the light in flecks in the green, so that the leaf has a softly mottled appearance. On the under side the light parts are a little greener than the upper as if the chlorophyll persisted longer on the under side. The palisade on the lighter parts does not lose its characteristic structure but only its colour.

In some other plants the variegation takes the form of white •patches, with very sharply defined edges because the chlorophyll is absent throughout the section. So that the patch is pure white from either side. This occurs for instance in a form of Anthurium Sp. where the white spots are characterised not only by the almost complete absence of chloroplasts, but also by the absence of all differentiation of tissue, the cells being nearly isodiametrie. (PI. II, Fig. 1.)

In Marajita vittata variegation takes the form of narrow white streaks between and parallel to the nerves which run acutely from the mid-rib, only on the upper side. But the palisade tissue loses not only its chlorophyll but also its characteristic form and consists of rounded cells indistinguishable from the spongey parenchyma, except for being colourless. The cells of the water-storage tissue of the upper side are here smaller, and the whole section somewhat thinner. (PI. I, Pig. 1.)

In other species are indications of the existence of a middle tissue, between the upper (palisade) and lower (spongey) tissues of the mesophyll. Thus in a common garden Dracaena, round ill-defined spots of slightly lighter green, about 1/16 in. diameter occur here and there in the leaf. A transverse section shows the normal mesophyll to comprise a layer of obviously elongated cells (the palisade), a second layer of much shorter cells and five or six layers of loosely

2563—42 arranged cells. In the lighter coloured spots a few layers in the middle of the spongey part are without chlorophyll (PI. II, Fig. 2). The effect of the absence of colour in this deep seated portion is to produce lighter spots with hazy, ill-defined outlines.

A variegated form of Alocasia macrorhiza, Schott, said to have arisen as a sport, affords a most interesting variability in the distri- bution of the chlorophyll — deficient tissue. This is always quite definite (as regards each layer of the leaf) without any merging as in Pothos aurea, or haziness as in the Dracaena. Sometimes a plant will give several pure white leaves which, however, soon wither, and are succeeded by more normal ones, or else the plant of course even- tually dies. Sometimes one whole half of a leaf is pure creamy white, without any trace of chlorophyll, while the other half is mottled. More usually the whole leaf is mottled as shown in Plate III. The mottling is in various degrees of lightness, and different on the two sides (compare the upper and under sides of the leaf in the plate).

Sections through different patches show that three regions can be recognised in the mesophyll, an upper (the palisade) a middle and a lower (the spongey-parenchyma). Writing the colours of these three regions in order from the top so that W.G.W. indicates white palisade, green core' and white spongey-parenchma, We find the following combinations to occur : —

G. G. G. G. G. W.

W. G. G. G. W. W.

W. G. W. W. W. W.

W. W. G.

It will be seen that only one of the eight possible combinations is absent, viz., G.W.G., or white core and fully-green cortex.

Though the spongey-parenchyma is thicker than the core, the chlorophyll, is less dense, and in a leaf held up against the light, patches of W.G.W, appear to be nearly of the same tint as those of W.W.G. Again when viewed by reflected light only, patches of G.G.W. appear the same as those of G.W.W., because of the much greater intensity of the green in the palisade tissue than in the middle region, but by transmitted light they are distinguished at once except in the Dracaena referred to above, so far I have not found G.W.G., which would be r ever son, in Bateson's sense of W.G.W.

A variegated Eranthemum Sp. shows the third middle layer even more clearly because the cells are coloured distinctly blue. This causes the leaf to have a dark bluey green colour except round the margin and in patches where the normal green or a lighter colour

occurs. Where a vascular bundle traverses the mesophyll the blue

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layers appear to expand and embrace the bundle, but to lose its

distinctive blue colour.

Breynia rhamncides a wild species common in Madras, affords another and curious type of variegation. In this the leaves, especially towards the ends of the branchlets are mostly white with dark green patches on the upper side and lighter green ones below. The upper and lower patches do not cover each other and a leaf seen against the light shows sis different shades of green. , The dark green patches of the upper side may be seen even with the naked eye to be definitely raised above general white surface and are due to the presence there of the- paliside tissue which is entirely absent elsewhere. Lighter patches on the lower side are due to the absence of chlorophyll in the spongey parenchyma, which, however, unlike the palisade, is of normal structure throughout. In addition to the two departures from the normal structure there is again, as in the variegated Alocasia a middle layer which, usually green, is white in some patches. Using the same notation as with Alocasia but marking — when a tissue is absent altogether, we find the following variegations to occur : —

G. G. G. G. G. W. G. W. W.

— G. G. — W. G. — W. W.

— G.W.

Conclusion

Variegated leaves of commonly cultivated plants in Madras show that very frequently there are in the leaf not two kinds of mesophyll- tissue as usually stated, but three. And since the chlorophyll may be present or absent independently in these, they appear to be funda- mentally distinct. Though in most plants examined, the middle layer was never found to be white unless one or other of the other two layers are, in Dracaena the middle layer is white in circular patches while the rest is green.

This has the appearance of the green skin over the white core of Enonymus Sp. etc. (Bateson I.e. p. 96), but the sub-epidermal white skin, as distinct from the palisade and spongey tissues as illustrated by Bateson (I.e.) has not been met with.

Explanation of Plate II

Fig. 1. Section through part of the variegated leaf of Maranta vittata, showing normal green tissue on right of a white streak.

Fig. 2. Section* through leaf of Dracaena S2J. Showing absence of chlorophasts in the middle region.