Lallands (Lowlands) Scots has its historical roots in the same Germanic source languages as English and seems to have diverged from Northumbrian Middle English after the 14th century. Those looking for a truly Scots language have Gaelic. However Lalland (and Doric in the north-east of Scotland) are overwhelmingly agreed to be dialects of English and the few who disagree seem politically motivated rather than basing their views on linguistic evidence.
Once the language of south and eastern Scotland it absorbed English vocabulary under the influence of James Vi who became James I of England and subsequently under the Act of Union (1603).
It was adopted by literary figures in the 18th century and so could be read in the works of Allan Ramsay, Robert Burns and Walter Scott. From the 19th century it can be read in the writing of Robert Louis Stevenson and J M Barrie and in the 20th in the poetry of Hugh McDiarmid (Christopher Murray Grieve).
Even more accessible are the comic creations Oor Wullie (left) and The Broons (above) drawn by Dudley Watkins and published in Scotland in The Sunday Post.
Using Scots dialect the cartoon strips are understandable (with a little extra knowledge of vocabulary) by English-speaking readers and rely considerably on vowel changes (more becomes mair), and other minor pronunciation divergences from RP written approximately phonetically to reflect the accent.
(http://www.putlearningfirst.com/language/12dial/scots.html)
Anna