As most of you know, most of our research recipies require the earlier version of the spell (Lesser summoning requires minor summoning, summon coldstone requires summon heatstone, etc.). This usually isn't a problem, except there are some times when to research a summon elemental spell, you also have to research the previous 1, 2, or 3 levels of that spell, because it can't be bought on vendors. This is usually called "Chain" researching. This page lists all the spells that can only be researched, that require "chain" researching.

Greater Summoning: Water (level 29) requires Summoning: Water (level 24)
Conjuration: Earth (level 44) requires Lesser Conjuration: Earth (level 39)
Greater Conjuration: Mist (level 49) requires Conjuration: Mist (level 44)
Greater Conjuration: Water (level 49) requires Conjuration: Water (level 44), which requires Lesser Conjuration: Water (level 39)

Of course, you can Chain research all of the spells if you want to, but these are the only places where you are forced to, because the lower level spells are not sold anyplace on Norrath.