On the nature of nature spirits, where the idea might have come from of tiny invisible beings responsible for the growth of plants, et al.

W. Y. Evans Wentz, The Fairy Faith in Celtic Countries:

In the positive doctrines of mediaeval alchemists and mystics, e.g. Paracelsus and the Rosicrucians, as well as their modern followers, the ancient metaphysical ideas of Egypt, Greece, and Rome find a new expression; and these doctrines raise the final problem—if there are any scientific grounds for believing in such pygmy nature-spirits as these remarkable thinkers of the Middle Ages claim to have studied as being actually existing in nature….

These mediaeval metaphysicians, inheritors of pre-Platonic, Platonic, and neo-Platonic teachings, purposely obscured their doctrines under a covering of alchemical terms, so as to safeguard themselves against persecution, open discussion of occultism not being safe during the Middle Ages, as it was among the ancients and happily is now again in our own generation….

All these Elementals, who procreate after the manner of men, are said to have bodies of an elastic half-material essence, which is sufficiently ethereal not to be visible to the physical sight, and probably comparable to matter in the form of invisible gases. Mr. W. B. Yeats has given this explanation:—’Many poets, and all mystic and occult writers, in all ages and countries, have declared that behind the visible are chains on chains of conscious beings, who are not of heaven but of earth, who have no inherent form, but change according to their whim, or the mind that sees them. You cannot lift your hand without influencing and being influenced by hordes. The visible world is merely their skin….’ [From Yeats’ Irish Fairy Tales and Folk-Tales]

Wentz again three paragraphs on:

And independently of the Celtic peoples there is available very much testimony of the most reliable character from modern disciples of the mediaeval occultists, e.g. the Rosicrucians, and the Theosophists, that there exist in nature invisible spiritual beings of pygmy stature and of various forms and characters, comparable in all respects to the little people of Celtic folk-lore.

Yeats’s words do somewhat remind me of the famous opening of the Reverend Robert Kirk’s Secret Commonwealth, wherein he says these beings

are said to be of a midle Nature betuixt Man and Angel, as were Dæmons thought to be of old; of intelligent fluidious Spirits, and light changable Bodies, (lyke those called Astral,) somewhat of the Nature of a condensed Cloud, and best seen in Twilight. Thes Bodies be so plyable thorough the Subtilty of the Spirits that agitate them, that they can make them appear or disappear att Pleasure. Some have Bodies or Vehicles so spungious, thin, and delecat, that they are fed by only sucking into some fine spirituous Liquors, that peirce lyke pure Air and Oyl…