How to Recruit an Army

Plants primarily secrete nectar as an energy source to tempt pollinators to visit their flowers, but the secretion of this substance appears to have evolved long before flowering plants appeared. Many plants, including some ferns, secrete nectar from extrafloral nectaries - i.e. nectaries in other positions on the surface of the plant. 

Legumes, like the common vetch Vicia sativa in the image above, have extrafloral nectaries on their stipules (the small, leaf-like projections on either side of the base of a leaf stalk). The extrafloral nectary is the black spot on the image above and a closer look ....

... reveals what its function might be. Ants are famous for their attraction to sweet substances and regularly visit the plant for the sugar that leaks out of these locations. This might deliver two kinds of selective advantage to the plant that would outweigh the cost of using some of its assimilated sucrose in this way. In some plants it might deflect ants, which are usually very inefficient pollinators, away from the larger source of nectar that's there to service more efficient pollinators, like bees. In other plants it may be a way of recruiting  a defensive army of ants because they become aggressive towards herbivorous insects that might try to plunder their food supply; in Acacia trees for example, the defensive benefits of hosting ants are well documented.

Extrafloral nectaries are found in a wide variety of plants and are often located on leaf petioles and mid-ribs. This is a vertical section through an extrafloral nectary on the underside of the mid-rib of a cotton plant (Gossypium sp.), stained with fluorescent dyes. The bright yellow cells at the top are xylem vessels, conducting water to the leaf blade. The very small, brick shaped blue cells below are dividing cambial cells and also phloem sieve elements that are conducting assimilated sucrose away from the leaf blade. Below that are some larger, blue-stained parenchymatous cells and then, at the very bottom, there are thin-walled finger-shaped cells which constitute the extrafloral nectary tissue, on the lower surface of the leaf mid-rib.

The blue staining is due to cellulose in the cell walls binding to a dye called calcofluor, which then fluorescence blue in UV light. You can see from this image that there's a very thin cellulose cell wall in those finger-shaped extrafloral nectary cells, because they barely fluoresce. So they easily leak sucrose that accumulates in them. The other interesting feature of this section is the orange staining in the small cells immediately above those extra-floral nectary cells. This is the endoplasmic reticulum/ Golgi complex inside the cells - the membranes and secretory vesicles that manufacture substances and transport them between cells via channels in the cell walls called plasmodesmata; these brightly-fluorescing cells seem to be highly metabolically active, so maybe the nectary cells are secreting something else, as well as sucrose.

There are some scientific papers on cotton extrafloral nectaries, their role and how they might be exploited in biological control programmes in this crop here, here and here.

Millions of Mites

By the time that summer arrives the foliage of most trees shows signs of insect attack, but these little eruptions on the surface of an alder leaf are caused by eriophyid mites, which are not insects but are related to spiders. I think the mite species that has produced these is Eriophyes laevis inangularis.

Each of these little domes is a chamber that's formed when the mites feed on cells on the undersurface of the leaf, leading to uneven growth that results in the formation of  a pouch where the mites can feed and breed.

This is the underside of the leaf, with the little yellow, sausage-shaped mites crawling around the entrances to the chambers, which are lined with nutritive cells that provide sustenance for the mites.

Here they are at higher magnification .........

............ and at still higher magnification, when the elongated body with four legs at the head end is visible in the mite in the top, left-hand corner. Each chamber is home to a brood of mites and a tree with a severe infestation could be covered with hundreds of thousands of them. Eriophyid mites also commonly infest sycamore and field maple leaves, producing large numbers of red pouches on the leaf surface.

These are three of the mites, each being about one fifth of a millimetre long, with only four legs.

The outer cuticle of the animal has a distinct pattern that differs between species, although the easiest way to identify species is via the symptoms that they cause on the host plant.

Here is the head, legs and cuticle patterning at higher magnification.

In addition to infesting sycamore, field maple and alder leaves eriophyid mites also attack many other plants, including goosegrass (aka cleavers) Galium aparine, whose growth is distorted by Eriophyes galii.

Typically, infested leaves curve inwards at the edges and become spoon-shaped, like the bottom, second-from-the-left leaf in this picture.

Here's the goosegrass eriophyid - the dark, globular structure top left is an air bubble on the microscope slide.

In this view you can see some of the surface patterning and an internal structure - perhaps an egg?- 

... and in this plane of focus the surface pattern of the cuticle is apparent.