now passed into common usage this distorted form of it $\surd$."]
This opinion was held generally until recently. The more careful study of German manuscript algebras and the first printed algebras has convinced Germans that the old explanation is hardly tenable. [...] The oldest of these is in the Dresden Library, in a volume of manuscripts which contains different algebraic treatises in Latin and one in German. [...] They [the main facts found in the four manuscripts] show conclusively that the dot was associated as a symbol with root extraction.
Christoff Rudolff was familiar with the Vienna manuscript which uses the dot with a tail. In his Coss of 1525 he speaks of the Punkt in connection with root symbolism, but uses a
  mark with a very short heavydownward stroke (almost a point), followed by a straight line or stroke, slanting upward. As late as 1551, Scheubel, in his printed Algebra, speaks of points.

See page 144 :

In 1553 Stifel brought out a revised edition of Rudolff's Coss. Interesting is Stifel's comparison of Rudolff's notation of radicals with his own, and his declaration of superiority of his own symbols. We read: "How much more convenient my own signs are than those of Rudolff, no doubt everyone who deals with these algorithms will notice for himself. But I too shall often use the sign $\surd$ [...]."

Added
From John Fauvel & Jeremy Gray (editors), The History of Mathematics: A Reader (1987).
Page 229 :

Al-Khwarizmi (ca.780 – ca.850) : "A square is the whole amount of the root multiplied by itself."

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