Concept:
Greenhouse structures are selected according to crop height, climatic condition, cost, and ease of construction.
Step 1: Lettuce is a low growing crop.
Step 2: For low growing crops, a simple and economical greenhouse structure is suitable.
Step 3: Hoop type greenhouse has a curved roof structure and is commonly used for low height crops.
Step 4: It is easy to construct and suitable for crops that do not require large vertical growing space.
Step 5: Hence, hoop type greenhouse is suitable for low growing crops such as lettuce.
\[
\boxed{\text{Hoop type}}
\]