Sub-class ii.—­Gymnosperms. 223.  Coniferae. 224.  Cycadeae.

Class ii.—­Monocotyledons. 2.  Cannaceae.  II.  AMOMALES. 34.  Liliaceae.  XI.  LILIALES. 41.  Asparageae.  Ditto 55.  Gramineae.  XV.  GLUMALES.

Sub-kingdom ii.—­Cryptogamic Plants.

1.  Filices.  I. FILICALES. 6.  Lycopodiaceae.  Ditto [page 69]

Radicles.—­In all the germinating seeds observed by us, the first change is the protrusion of the radicle, which immediately bends downwards and endeavours to penetrate the ground.  In order to effect this, it is almost necessary that the seed should be pressed down so as to offer some resistance, unless indeed the soil is extremely loose; for otherwise the seed is lifted up, instead of the radicle penetrating the surface.  But seeds often get covered by earth thrown up by burrowing quadrupeds or scratching birds, by the castings of earth-worms, by heaps of excrement, the decaying branches of trees, etc., and will thus be pressed down; and they must often fall into cracks when the ground is dry, or into holes.  Even with seeds lying on the bare surface, the first developed root-hairs, by becoming attached to stones or other objects on the surface, are able to hold down the upper part of the radicle, whilst the tip penetrates the ground.  Sachs has shown* how well and closely root-hairs adapt themselves by growth to the most irregular particles in the soil, and become firmly attached to them.  This attachment seems to be effected by the softening or liquefaction of the outer surface of the wall of the hair and its subsequent consolidation, as will be on some future occasion more fully described.  This intimate union plays an important part, according to Sachs, in the absorption of water and of the inorganic matter dissolved in it.  The mechanical aid afforded by the root-hairs in penetrating the ground is probably only a secondary service.

The tip of the radicle, as soon as it protrudes from the seed-coats, begins to circumnutate, and the whole

* ‘Physiologie Végétale,’ 1868, pp. 199, 205. [page 70]

growing part continues to do so, probably for as long as growth continues.  This movement of the radicle has been described in Brassica, Aesculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea.  The probability of its occurrence was inferred by Sachs,* from radicles placed vertically upwards being acted on by geotropism (which we likewise found to be the case), for if they had remained absolutely perpendicular, the attraction of gravity could not have caused them to bend to any one side.  Circumnutation was observed in the above specified cases, either by means of extremely fine filaments of glass affixed to the radicles in the manner previously described, or by their being allowed to grow downwards over inclined smoked glass-plates, on which they left their tracks.  In the latter