In the passages which denounce the ability of the Old Masters to faithfully portray trees, Ruskin consistently turns to the most outlandish metaphors available at the time, in order to emphasise the gulf which he perceived between the imagined nature of studio artists and the real landscape from which Ruskin draws notions of truth in representation.
In this case, Ruskin 's use of marine monsters offers a suitably grotesque image to counterpoint the natural grace he finds in real trees, and it may be the case that he was drawing on his recent reading in fossil ichthyology. During his time recuperating at Dr Jephson's in 1843, Ruskin selected from the local library, Louis Agassiz's Poissons Fossiles, which dealt in detail with the fossil remains of previous oceanic life, including many species, such as the plesiosaur, which to Victorian eyes appeared distinctly monstrous ( Works, 1.457).