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Show 552 SUl\ilVIAH.Y AND CHAP. XII. from it. Whether this can be of any service to them is very doubtful, but with seeds germinating on the surface it will slightly aid geotropism in directing the radicles to the ground.* We asccrtaine<l in one instance that such sensitiveness resided in the tip, and caused the adjoining parts to bend from tho light. The sub-aerial roots observed by vViesner wore all apheliotropic, and this, no doubt, is of use in bringing them into contact with trunks of trees or surfaces of rock, as is their habit. We thus see thn,t with seedling plants the tip of the radicle is endowed with diverse kinds of sensitiveness; and that the tip directs the adjoining growing parts to bend to or from the exciting cause, according to the needs of the plant. The sides of tho radicle are also sensitive to contact, but in a widely different manner. Gravitation, though a less powerful cause of movement than the other above specified stimuli, is ever present ; so that it ultimately prevails and determines the down ward growth of the root. The primary radicle emits secondary ones which project sub-horizontally; and these wore observe~ . in one case to circumnutate. Their tips are also sensitive to contact, and they are thus excited to bond aw~y from any touching object; so that they resemble m these respects, as far as they were observed, the primary radicles. If displaced they resume? ~s Sachs has shown, their original sub-horizontal position; and this apparently is due to diageotropism. The secondary radicles emit tertiary ones, but those, in the case of the bean, are not affected by gravitation; consequently they protrude in all directions. Thus the general * Dr. Karl Riohter, who has especially attenrled to this subject (' K. Aknd. der Wissenscha.tten in Wien,' 1879, p. 140), stat~s th:t apheliotropism dors not atd r • dicles iu penetrating the ground. CIJAP. XII. CONCLUDING H.El\1ARKS. 553 arrangement of the three orders of roots is excell tl adapted for seaJC. hI' ng t h e whole sm. l for nutrimenet.n y ~achs. has shown that if t~he t~p of the primary radicle IS cut off (and the tip will occasionally be gnawed off with se~dlings in a state of nature) one of the se~ondary radiCles. gr~ws perpendicularly downwards, m a manner whiCh IS analogous to tho upward growth ?f a lateral shoot after the amputation of the leadmg shoot. We have soon with radicles of the ~an that if ~he primar! radicle is merely compressed mstead of bemg cut off, so that an excess of sap is directed into ·the secondary radiclcs, their natural condition is disturbed and they grow downwards. Other analogous facts have been given. As anything which disturbs the constitution is apt to lead to reversion that is, to the resumption of a former character, i~ appears probable that when secondary radicles grow downwards or lateral shoots upwards, they revert to the primary manner of growth proper to radicles and shoots. With dicotyledonous seeds, after the protrusion of the radicle, the hypocotyl breaks through the seedcoats; but if the cotyledons arc hypogean, it is the epicotyl which breaks fm·th. These organs are at first invariably arched, with the upper part bent back parallel to the lower ; and they retain this form until they have risen above the ground. In some cases, however, it is the petioles of the cotyledons or of the first true leaves which break through the seed-coats 118 well as the ground, before any part of the stem protrudes; and then the petioles arc almost invariably arched. We have met with only one exception, and that only a partial one, namely, with the petioles of the two first.leaves of Acanthus candelabr'um. With Delphinium nudwaule the petioles of the two cotyledons are com- |