As a non-native English speaker, I have a bit of trouble finding the plural of the word 'lettuce'. In my own language (Dutch) it doesn't have a plural at all, and 'lettuces' simply sounds funny to me. Is that just a feeling, or is this word actually not really being used? Would there be another way of expressing the same thing (i.e. bunches/pieces of lettuce), what would be the word of quantity for lettuce?
In British English, to say lettuces (and to speak of cabbages and kings and so forth) is entirely conventional. whereas in American English, lettuce is typically uncountable, and lettuces only used when referring to different types of lettuce, in the same way we can say peoples or cereals.
For natural units of lettuce, we would say
- heads of lettuce — for multiple specimens of an entire (iceberg or Romaine-type) lettuce plant.
- leaves of lettuce or simply pieces of lettuce — for the leaves pulled off of the head.
[aside: despite BrE being okay with pluralizing lettuce, it seems that what Americans know as mashed potatoes is called mashed potato, and, n.b. to the marketing director, to see it advertised as such in the hotel restaurant's Thanksgiving Day special makes the Yankee visitor more homesick, not less].
For many fruits and vegetables, the name for the food as a mass noun corresponds to the name for an individual example of it. A recipe might instruct you to add one grated carrot or to add 100cc of grated carrot. You can slice up half a pineapple to add more pineapple to a fruit salad.
For vegetables where only the leaves (lettuce, kale, arugula, broccoli, watercress, endive, collard greens, spinach, chard, cauliflower, escarole, cabbage, mustard greens, etc.) or stalks (asparagus, celery, rhubarb) or other sub-components are consumed, however, there is no such correspondence. We ask for bunches of bibb lettuce or watercress (a conventional supermarket bunch measuring around 1 pound) or heads of cabbage or broccoli; and ears of corn (maize) as Gary's Student has noted.
Where the consumed specimens are granular and consumed en masse, the foodstuff is pluralized— an individual pea, but two cups of peas, not pea, and the same for foods like lentils and beans, berries, scallions, leeks, and smaller mushrooms.
This may be analogized to meat; meat, seafood, and poultry are generally mass nouns (lamb, squid, pheasant) unless speaking of an entire animal (ten oxen, six octopuses, fifty quail).
Garlic is one exception (I am sure there are others). It is sold in bulbs consisting of cloves, but neither unit would be pluralized as garlics.
The usual way (US) of indicating more than one plant:
"There are five heads of lettuce on the table."
In a similar fashion:
"There are five ears of corn on the table."
In Britain we would say 'there are five lettuces on the table'.
I can think of circumstances where growers might talk of 'lettuce' as though it were not a count noun - 'we have raised more lettuce than we did last year'. Or even caterers might say 'we will need a lot of lettuce for the salad'.
But generally it is 'lettuces'.
This is just like fish and fishes, deer and deers, etc.
The word lettuce is an uncountable quantity - measured and not counted:
Have some lettuce. Johnny has more lettuce. (Not Have another lettuce or Johnny has 3 lettuces and I have only 2.)
Joe Hunter shot 14 deer today. (Not He shot 14 deers today.)
However, the word lettuces exists, and refers to multiple kinds of lettuce.
Romaine lettuce, bib lettuce, red-leaf lettuce, and butter lettuce are all lettuces.
Wapiti, moose, and white-tailed deer are all deers. (They are all members of the deer family.)