There's always a corner in the garden for one or two lettuces.

Lactuca sativa comes in countless forms - colours range from green to red and can have yellow, gold or blue-teal leaves. The 18th-century Scottish gardener, James Justice, even referred to a variety, Aleppo, which was valued for its "fine spotted leaves", like the modern variety Freckles. Some varieties, Cos or Romaine types, have wonderful, crisp leaves; crispheads and butterheads heart up well; while cut-and-come-again varieties add a touch of freshness to any sandwich.

Red-leafed varieties have become very popular. Antioxidant anthocyanins make vegetable leaves red, so there are health reasons for growing and eating red veg - as well as the fact they look attractive when planted beside green ones.

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These anthocyanins also affect how a plant photosynthesises: among other things, red lettuces need less water. So, during a hot, dry spell, they should fare better than their green counterparts. They may also be a weapon in the battle with slugs. The garden's top pest hates the wonderful dry weather we all adore, but it's also been suggested that it finds red-leaved vegetables less palatable.

I'm no fan of slugs, so I'm going to see if there's any truth in the theory that they find the slightly more bitter red leaves less appealing. It would also be interesting to see if the type of lettuce, rather than its colour, is important. I've sowed Rubens Red, a large Cos-type with dark-green leaves overlaid with red, and will compare its life expectancy with a green variety, Buttercrunch. Do the slugs keep snug inside a red or green heart, or both?

If slugs are more exposed when chomping loose-leaf varieties, do they risk their lives on a Red Batavian or the green oakleaf variety Catalogna? It will be amusing to see if colour matters to molluscs. I suspect they'll make do with Rubens Red if that's all there is.

Like slugs, humans have had a long history of enjoying lettuce, dating back to the Ancient Egyptians. At first, they made oil from lettuce seed, but by 2680BC they were breeding selectively for larger and better leaves. Lettuce also had religious connotations and was used in sacrifices to the god, Min. According to George Hart's Routledge Dictionary of Egyptian Gods And Goddesses, Min "performed the sexual act untiringly" after consuming lettuce. More mundanely, the Egyptians first bred the Romaine or Cos lettuce which was later adopted by Greeks and Romans.

By the 16th century, gardeners in mainland Europe were planting the three main types of lettuce we eat today. In 1586, the German academic and botanist Joachim Camerarius described heading lettuce, loose-leaf and Romaine or Cos ones. When Justice was writing 200 years later, he listed 10 varieties, including an Egyptian green Cos.

Justice reckoned that, while there were several more types on the market, his 10 were the only ones worth considering. But Victorian seedsmen thought differently, bombarding gardeners with a bewildering choice every year, believing new was always best. They didn't seem to care whether there was any difference between the old and the new and probably gave the same variety several different names.

This does make me wonder if we could be in danger of playing the Victorian game. Perhaps the European Register of seed varieties is not as bad as it's often portrayed. Custodians of heirloom varieties complain that they can't afford to register heritage varieties and that only big seed companies have funds to do this. The genetic pool is thereby diminished by commercial interests. Of course, this is partly true. Gardening fashions change and some interesting old varieties could sadly be lost. But it's also possible that some varieties may be known by different names and the genetic pool isn't as large as is often claimed.