[524] Mr. Selby, in his 'History of British Forest Trees,' p. 465, gives the following account of the formation of this peculiar growth:—"In the autumn the parent aphis deposits her eggs at the base of the embryo leaves, within the bud destined to produce the shoots of the following year. When these begin to burst and expand in spring, the leaves, at whose bases the eggs have been deposited, instead of increasing in length, enlarge at the base, and form a cell or cyst whose mouth is at first closed by a red velvety-looking substance. If opened in this state a nest of small greenish aphides is distinctly visible, and at a certain period, or when they have acquired maturity, which is towards the end of the summer, the mouth of the cell opens and the insects fly off to inflict a similar injury upon the nascent buds of the year. In some instances the leaves of only a portion of the circumference of a shoot are affected, in which case, though a slight distortion may take place, the branch is not prevented from elongating; but in others, where the whole of the leaves around the shoot are converted into nidi, elongation is prevented and distortion to a great extent takes place."
[525] See Cramer, 'Bildungsabweich.,' pp. 53, 64, for further references.
[526] 'El. Ter. Veg.,' p. 124.
[527] Schlechtendal, 'Bot. Zeit.,' 1857, vol. xv, p. 67.
[528] On the subject of this paragraph the reader may consult A. Braun, "Ueber abnorme Blattbildung," &c., in 'Verhandl.,' d. 35, 'Naturforscherversammlung;' Jaeger, 'Flora,' 1850. p. 481, tab. 4, Digitalis.
[529] 'Org. Veget.,' i. p. 286.
[530] 'Bull. Soc. Bot. France, vol. viii, 1861, p. 710.
[531] 'Linnæa,' 1830, vol. v, p. 492.
[532] 'Mus. Senkenb.,' ii. p. 45.
[533] 'Bull. Acad. Roy. Belg.,' 1851, t. xviii. part i, p. 275.