Aphids in a Savage Landscape

When aphids infest plants they tend to find a good spot to feed and then stay in one place, where they'll insert their stylets into the plant's phloem, tap its sugary sap and then settle down to reproduce. 

When you take a close look at plant surfaces you can sometimes see why these pests are more or less sedentary. Many plants, like this goosegrass Galium aparine, are covered with epidermal hairs (trichomes) that make it difficult to tiny aphids to move around.

In the case of goosegrass the hooked hairs are primarily for attaching their weak stems to supports as they grow, but those curved spines are also awkward obstacles for minute aphids to negotiate.

Hooked on Hops

Hops Humulus lupulus have an impressive ability to climb supports - either up other plants or, in the case of cultivated hops, up poles in hop gardens. Charles Darwin devoted a lot of time to studying the way in which their shoot tips rotate as they grow (by the process of circumnutation), seeking out objects to coil around (you can read more about his experiments here). There's more to hops' climbing ability than circumnutation and rapid growth, however - their stems are clothed in very distinctive epidermal hairs (trichomes) that act as grappling hooks, securing their grip on supporting structures.

 

The hop trichomes that are adapted for climbing have a very distinctive anvil shape - you can see them here, at low magnification, on either side of a hop leaf petiole.

At higher magnification the anvil shape is very distinctive, something noted ....

.... by the botanist Anton Kerner von Marilaun in his Natural History of Plants (1895).

Hops have been cultivated for centuries, primarily for the resins produced by their epidermal glands, mainly at the base of the bracts in the female flowers but also on other parts of the plant, including the underside of the leaf. In the photograph above you can see the minute gold drops of resin on the lower surface of a hop leaf. The resins are converted to bitter isohumulones during the brewing process, adding a distinctive flavour to beer.

The Colour Purple

This rather beautiful flower is Tibouchina urvilleana and the purple of its petals is due to the presence of anthocyanin pigments, which are dissolved in the cytoplasm of the petal cells.

If you magnify the petal surface about 200 times you can see the way in which the petal cells fit together, like pieces of a jigsaw puzzle. This piece of petal is mounted in water but if it's transferred to a concentrated solution of sugar ...

  ... the water in the cell begins to flow out through the semi-permeable cell membrane by the process of osmosis, with water travelling out from the less concentrated solution in the cell to the more concentrated solution surrounding it. Within a few minutes spaces become visible between the cytoplasm and the cell wall, where the cytoplasm shrinks ....

... and within a few more minutes the cytoplasmic contents of the cell have shrunk even further, so the purple anthocyanin pigment becomes even more concentrated in the remaining cytoplasm.

  While I was looking at the petals I noticed something unusual around their edge  - a fringe of microscopic hairs, invisible to the naked eye.

 Each hair is tipped with a  glandular head.



... that looks as through it may contain oils.

What are these hairs for? Secreting aromatic compounds that attract insect pollinators, perhaps?

Defensive Weapons

The outer layer of cells on a plant's surface - the epidermis - is the first line of defence against herbivores, pests and diseases so it's not surprising that many plants are covered with an array of defensive weapons. Sometimes these are cells that secrete repellent biochemicals, which give many plants a characteristic aroma when you brush their leaves. Other species have mechanical barriers, in the form of dense coverings of hairs (trichomes) to deter small insects like aphids. Stinging nettles are covered in a forest of complex stinging hair cells, each mounted on a pediment of cells. You can see some further, more detailed images of the structure of the stinging hairs here, but the image above is an aphid's-eye view of a nettle leaf underside - although they wouldn't see it in these lurid colours, which I generated using polarised light.

Fragrant Foliage

If you’ve ever brushed against a scented-leaved Pelargonium like the one above (or any other fragrant-leaved plant) and noticed the scent, here’s where that fragrance is actually stored. The leaves are covered in fine pointed hairs (second photo from bottom) but scattered amongst them there are two other kinds of hair – short (third from bottom) and tall (fourth photo from bottom). At higher magnification (top photo) you can see the oily fragrance compounds stored in the bulbous cell at the tip of the hairs, that are released when the hairs are damaged. These fragrance chemicals are a key part of the plant’s defence system against insects and are secretions that either make the leaf too sticky for small insects to move around or repel them, but commercially they are of immense importance in the fragrance industry. Many of those scents that saturate the air around the perfume counter in department stores contained chemical compounds distilled from the ‘gunk’ that you can see in these plant hair tips. Perhaps the most extraordinary is labdanum, which is stored in the sticky hairs of Cretan rock roses (Cistus species). Tradionally, this was once collected by scraping it off the fur of goats that grazed amongst the Cistus shrubs on Cretan hillsides. You can read about it at http://botano.gr/herbs-and-spices/cistus-creticus-labdanum.html

Umbrellas to keep the water in

The leaves of the oleaster (Elaeagnus sp.) shrubs in my garden are glossy green above and dazzling white below. The cause of the highly reflective undersurface is revealed under the microscope – thousands of overlapping, flattened hairs, shaped like multi-armed starfish. Each is about a fifth of a millimetre in diameter and attached to the leaf surface by a short stalk. Imagine a surface covered in vast numbers of overlapping, flat, open umbrellas and you’ll have a pretty accurate mental picture of how they’re arranged. The hairs prevent excess water loss from the pores (stomata) on the leaf undersurface, while allowing free passage to the all-important carbon dioxide that the leaf needs for photosynthesis.