Size

• For the most part, orange trees grow bigger than lemon trees in all respects. Orange trees grow up to 25 feet tall, with some older trees in existence reaching 50 feet tall. Lemon trees usually max out at 20 feet tall. An orange tree produces oranges between 2 1/2 to 6 inches long and 1 to 3 3/4 inches wide. Comparatively, a lemon tree develops lemons around 2 3/4 to 4 3/4 inches long.
  
  

Variations in Classification

• Two major classifications--sour and sweet--of oranges exist. Each species of sweet orange belongs to one of four classifications, based on shape, color and acidity. The categories consist of round, navel, pigmented or blood oranges, and acidless oranges. Lemon trees generally come in three varieties--true, Meyer and Ponderosa--with many species of lemon trees embedded in those types. The first variety, true lemons, developed in India, spread into the Mediterranean Basin by unknown means, and crossed the Atlantic with help from Columbus. The eureka and Lisbon species define two separate branches of true lemons, characterized by tree shape, number of seeds, and season of fruit production. Ponderosa lemons, a citrus fruit with the appearance of true lemons, developed accidentally from a seedling in the 1880s.
  
  

Hybridization

• The Meyer lemon, considered a lemon despite its status as a hybrid, possesses less acid than true lemons. Meyer lemons look more like a yellow orange than a true lemon. In fact, genetic material from mandarin orange and lemon trees created the Meyer lemon. In 1908, an explorer named Frank N. Meyer found the tree planted in a pot in Peking, China.
  
  

Harvesting

• Lemons bruise easily if mishandled. Thus, human harvesters handpick lemons. Choosing when to harvest a lemon depends on the juice content--the fruit needs to be at least 25 percent juice. Lemons usually cure in transit, turning from green to yellow. Oranges, on the other hand, respond well to machine picking, though cost keeps the practice from being widespread.
  
  

Quality Control

• The U.S. Department of Agriculture, individual state citrus commissions and federal regulations determine the quality of oranges used in commercial operations and the fruits' final destination in produce departments or as processed food items such as juice. Lemon harvesters discard fruit less than 2 1/8 inches and grade the remaining fruit for processing or fresh use.