in Thoreau’s Journal:

So too the green veils, or screens of the birches rapidly thickened….It is the first to clothe large tracts of deciduous woodlands with green-& perchance it marks an epoch in the season. the TRANSITION decidedly and generally from bare twigs to leaves.

When the birches have put on their green sacks then a new season has come.— The light reflected from their tender yellowish green is like sunlight.