'Grass' is natural truth, and 'flower of the field' is spiritual truth, as in Isaiah 40:5, 6. Green grass, in the Word, signifies the good and truth of the church, and of faith, which first springs up in the natural man. This is also signified by 'herb in the field.' Grass and the pulse of the herb, as in Psalm 32:2, signify what is most vile. Grass signifies science from a spiritual origin, or that by which spiritual truth is confirmed. 'Reeds and rushes' signify science from a sensory origin, or the things which confirm the fallacies of the senses, as in Isaiah 25:7. This science, considered in itself, is only a part of the lowest natural degree, which may be justly called material and bodily, in which there is little or no life. Green grass also signifies that which is alive with man. 'Grass burnt up' signifies the things which are dead in him. Grass signifies the scientific principle. Just as green grass serves animals for support, spiritual truth serves men for spiritual nourishment.

(References: Apocalypse Explained 507)

528. Verse 13. And I beheld, and I heard one angel flying in the midst of heaven, saying with a great voice, Woe, woe, woe, to them that dwell on the earth, by reason of the other voices of the trumpet of the three angels which are about to sound!- "And I beheld, and I heard one angel flying in the midst of heaven," signifies the Lord enlightening all in the heavens concerning the state of the church at its end; "saying with a great voice," signifies in manifestation; "Woe, woe, woe, to them that dwell on the earth by reason of the other voices of the trumpet of the three angels which are about to sound," signifies grievous lamentation over the changes of the state of the church, on account of the turning away from good and truth, and thence damnation.