Don't know what type it is but you waited to long to harvest it. Your lettuce has bolted (gone to flower) and when that happens the lettuce will be bitter. Don't know what zone you're in but it was probably planted at the wrong time as heat causes it to bolt. For future reference, you plant lettuce in the early spring and in the fall. And you can harvest lettuce at any size, you cut individual leaves when young or harvest the whole plant when larger.

Rodney

Correct on every point.

Lettuce, depending on variety, can be grown all year long. Plan on sowing a little every month, because during the summer it will tend to bolt and you will have to use it quickly before that happens. In the spring and fall it will resist going to seed, and this makes life a bit easier. A great variety is Bibb, but some of the leaf lettuces are slower to bolt in the summer. There are definitely some tricks to keep the lettuce from bolting. The tips I offer here were taken from the techniques illustrated and described at http://alan-chadwick.org/html%20pages/techniques/garden_plants/veg_photos.html
1. Sow in seed flats and plant out when about 2" tall.
2. Avoid getting water on the leaves of the new transplants for 3 days or so, as this stimulates the plants to make leaf growth. What you want at this stage is root growth.
3. Plant close enough so that the leaves just touch each other from adjoining plants when full grown. This creates a zone of thermal control over the soil surface that keeps in moisture and coolness in the heat of the day.
4. Much more information is available at http://www.alan-chadwick.org

Here is a link that might be useful: Alan Chadwick

Many thanks to all. I live in Massachusetts.

Again, thank you!

ronjason, I'd be saving the seed and planting it in the spring. Lettuce seeds are usually true to the parent unless there's a lot of sow's thistle weed (wild lettuce) growing nearby. Really, you have nothing to lose if you sow it as soon as it is ready, you may have some young lettuce before freeze up.

ronjason, I'd be saving the seed and planting it in the spring. Lettuce seeds are usually true to the parent unless there's a lot of sow's thistle weed (wild lettuce) growing nearby. Really, you have nothing to lose if you sow it as soon as it is ready, you may have some young lettuce before freeze up.