No explanation has hitherto been attempted of such cases as the foregoing,- -namely, of secondary radicles growing vertically downwards, and of lateral shoots growing vertically upwards, after the amputation of

* ‘Proc.  Acad.  Nat.  Sc.  Philadelphia,’ June 16th, 1874, and July 23rd, 1875. ** See F. Elfving’s interesting paper in ’Arbeiten Bot.  Institut., in Würzburg,’ vol. ii. 1880, p. 489.  Carl Kraus (Triesdorf) had previously observed (’Flora,’ 1878, p. 324) that the underground shoots of Triticum repens bend vertically up when the parts above ground are removed, and when the rhizomes are kept partly immersed in water. [page 190]

the primary radicle or of the leading shoot.  The following considerations give us, as we believe, the clue.  Firstly, any cause which disturbs the constitution* is apt to induce reversion; such as the crossing of two distinct races, or a change of conditions, as when domestic animals become feral.  But the case which most concerns us, is the frequent appearance of peloric flowers on the summit of a stem, or in the centre of the inflorescence,—­parts which, it is believed, receive the most sap; for when an irregular flower becomes perfectly regular or peloric, this may be attributed, at least partly, to reversion to a primitive and normal type.  Even the position of a seed at the end of the capsule sometimes gives to the seedling developed from it a tendency to revert.  Secondly, reversions often occur by means of buds, independently of reproduction by seed; so that a bud may revert to the character of a former state many bud-generations ago.  In the case of animals, reversions may occur in the individual with advancing age.  Thirdly and lastly, radicles when they first protrude from the seed are always geotropic, and plumules or shoots almost always apogeotropic.  If then any cause, such as an increased flow of sap or the presence of mycelium, disturbs the constitution of a lateral shoot or of a secondary radicle, it is apt to revert to its primordial state; and it becomes either apogeotropic or geotropic, as the case may be, and consequently grows either vertically upwards or downwards.  It is indeed pos-

* The facts on which the following conclusions are founded are given in ‘The Variation of Animals and Plants under Domestication,’ 2nd edit. 1875.  On the causes leading to reversion see chap. xii. vol. ii. and p. 59, chap. xiv.  On peloric flowers, chap. xiii. p. 32; and see p. 337 on their position on the plant.  With respect to seeds, p. 340.  On reversion by means of buds, p. 438, chap. xi. vol. i. [page 191]

sible, or even probable, that this tendency to reversion may have been increased, as it is manifestly of service to the plant.

SUMMARY OF CHAPTER.