What's the meaning of **for** here?

In The Fairy Tale of Father Brown by G. K. Chesterton, the author was talking about a prince returning to his castle through the forest, saying:

“The moon had risen in strengthening silver, and the sky showed in stripes of bright, nocturnal blue between the black stripes of the pines about the castle. Flowers of some wide and feathery sort—for he had never noticed such things before—were at once luminous and discoloured by the moonshine, and seemed indescribably fantastic as they clustered, as if crawling about the roots of the trees.

What's the meaning of for here?

Solution 1:

For means seeing that or since here:

for, conj.
2. a. Introducing the ground or reason for something previously said: Seeing that, since. Cf. Greek γάρ, Latin nam or enim, French car, German denn.
_{Source: Oxford English Dictionary (login required)}

The flowers, rather than being called by their common or botanical name, are described, and that is because he had never seen them before and didn't know what to call them:

Flowers of some wide and feathery sort (no name for them—seeing that he had never noticed such things before) were at once luminous and discoloured by the moonshine . . .