System.Security.Cryptography.* are in different dlls?

I am constructing cryptohraphic app in VS 2010 Pro B2.

Classes from System.Security.Cryptography.X509Certificates
are resolved without adding any   references in  project
but  classes from 
System.Security.Cryptography.Pkcs.*
and 
System.Security.Cryptography.Xml.*
are being available only after explicit adding the reference to assembly
System.Security from GAC.

What are principles of having the cryptographic services components in different dlls in .NET? 
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