Some possible additional crosslinks
(in some cases I do not know which of two possibilities is better):
Land Tenure requires Law
Medicine requires Mathematics
Chemistry requires Gunpowder
Physics requires either Machinery or Hydraulics
Either Record Keeping or Writing requires Pottery (some of the first writing was on clay tablets.)
Civil Liberties requires either Constitution or Printing
? Dogma requires Politics?
Free Artistry requires either Artisanry or Architecture
Journalism requires Civil Liberties
? Flight requires Pneumatics?
? Firearms requires Military Conduct?
Labor Unions requires Civil Liberties
By the way explosives predate plastics by a good deal.
Maybe you should relabel the tech advanced explosives.
Not sure of the logic of Plastics requiring Refrigeration;
requiring Pharmaceuticals might be slightly better.
Possibilities for when the tech tree is eventually expanded:
Insurance, Telegraph, Canning (to preserve food), Horsecollar, Crop Rotation,
Modern Plow, Lightbulb, Glassblowing, Astrology or Stargazing, Rhetoric, Fermentation.
Here's the crosslinks I already have on my list for 0.9.3:
Philosophy ---> Aesthetics
Navigation ---> Compass
Machinery ---> Printing
Hydraulics ---> Sanitation
Nationalism ---> Military Science
Refrigeration ---> Plastics
Plastics ---> Explosives
Crosslinks will not show as arrows on the tech advisor screen but instead as an icon in the top right corner of the relevant tech.