Early Development Stages

• During germination the stem and seed leaves emerge, as on this bean .Green sprout image by Vladimir Krivsun from Fotolia.com
  
  Experts at the University of Arizona recognize three stages of development for tomato seedlings -- germination, post-emergence and transplant.
  
  

Germination and Post Emergence

• Tomato seeds germinate in 5 to 7 days.Cherry tomatoes image by Dmitriy Lesnyak from Fotolia.com
  
  Most tomato varieties germinate in five to seven days under proper conditions. Post-emergence occurs in five to 12 days if tomatoes are grown hydroponically, in 14 to 21 days if soil-grown.
  
  

Seed Leaves

• Seed leaves, like the ones on these sprouts, are directly opposed.sprouts image by Jesse-lee Lang from Fotolia.com
  
  Seed leaves (cotyledons) are the first leaves formed on a tomato and appear during the post-emergence phase. They are two single, smooth-edged, long and narrow leaves directly opposing each other. True leaves will sprout above the seed leaves.
  
  

True Leaves

• Tomato true leaves are lobed.tomatoe plant image by Coralie Palmeri from Fotolia.com
  
  True leaves are the second set of leaves developed. They are compound when fully developed, meaning they have more than one leaflet attached to the leaf stem (rachis). They differ from seed leaves in being lobed (notched edges).
  
  

Timing

• The average tomato seedling is 2 inches high when true leaves appear.tomato plants image by Gina Smith from Fotolia.com
  
  The first true leaves can appear on hydroponically grown seedlings at five to 12 days after germination. True leaves usually appear on soil-grown seedlings at two to three weeks of age (14 to 21 days). At this point, the average tomato seedling is about 2 inches high.