Every time I collect tomatoes in the garden, my hands end up covered with an invisible substance with a pungent smell. It seems to come from the tomato leaves and branches. When I wash my hands with soap, the substance becomes a very bright yellow-green 鈥� almost fluorescent 鈥� and it stains my soap, towel and wash basin. However, if I don鈥檛 use soap to wash the substance off, it remains invisible. What is it?
鈥� Leaves of plants in the family (including tobacco, tomato, potato and capsicum) all have minute hairs on their surface which exude drops of a sticky fluid.
The function of this sticky substance is not altogether clear, but it could ward off attacks from aphids and other sap-sucking insects. The flavonoids and possibly other pigments in the fluid react with soap, which is alkaline, and change colour accordingly.
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David Whitehead, Cape Town, South Africa
鈥� I remember being impressed long ago by the bright green colour that developed when washing my hands after helping my father tend his tomatoes. Some time later, as a botany student, I examined the tomato leaf epidermis under a stereo-microscope, with interesting results.
The tomato epidermis carries two types of multicellular hair 鈥� long ones of several millimetres readily seen with the naked eye, and much shorter hairs with four glandular sacs like short sausages at the apex. These are filled with khaki-coloured contents and have very thin, fragile cell walls. I found that prodding one of these sacs with a needle released sticky contents that could be drawn out as a thin thread which set rigid within about 2 seconds, leaving a deposit on the needle.
Later still, I found some photos of a mite wearing what can only be described as concrete boots. These were apparently made up of accumulated secretions from a tomato plant, collected as it rambled over the plant鈥檚 surface. The deposit clearly encased the mite鈥檚 legs, preventing it from hanging onto the leaf, thereby acting as a defence mechanism against this and other small, walking herbivores.
鈥淚 found some photos of a mite wearing what looked like concrete boots. These were made up of tomato secretions鈥�
Casual brushing against a tomato leaf will transfer only a little of the substance. This reserves the secretion for organisms that ramble among the hairs 鈥� or anything grasping the plant strongly enough to bend the longer hairs.
The deposit鈥檚 colour is lost against the skin of the average gardener, but it can certainly be detected by smell and, I suspect, by texture.
Jim Kent, Minehead, Somerset, UK