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.

Within Every Grass Leaf There Are Hidden Smiley Faces .....

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A vascular bundle in a transverse section of a grass leaf, stained with the fluorochromes Calcofluor M2R (blue fluorescence = cellulose) and auramine O (yellow fluorescence = lignified cell walls). The red fluorescence is chlorophyll autofluorescing red in the blue excitation beam of the microscope. 

The two big 'eyes' in this 'smiley face' (which is typical of a monocot vascular bundle) are metaxylem elements that transport water through the leaf. The bright blue fluorescence in the 'mouth' of the 'smiley face' is phloem, composed of larger sieve tubes and smaller rectangular (in cross section) companion cells, which together transport sugars, made by photosynthesis, out of the leaf. The bright yellow cells forming the neck of the 'smiley face' are lignified, providing a measure of rigidity in the leaf,  and the band of cells along the bottom of the section are epidermal cells covered by a cuticle.