We thus see that 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 the 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 downward growth of the root.

The primary radicle emits secondary ones which project sub-horizontally; and these were observed in one case to circumnutate.  Their tips are also sensitive to contact, and they are thus excited to bend away from any touching object; so that they resemble in these respects, as far as they were observed, the primary radicles.  If displaced they resume, as Sachs has shown, their original sub-horizontal position; and this apparently is due to diageotropism.  The secondary radicles emit tertiary ones, but these, in the case of the bean, are not affected by gravitation; consequently they protrude in all directions.  Thus the general

* Dr. Karl Richter, who has especially attended to this subject (’K.  Akad. der Wissenschaften in Wien,’ 1879, p. 149), states that apheliotropism does not aid radicles in penetrating the ground. [page 553]

arrangement of the three orders of roots is excellently adapted for searching the whole soil for nutriment.

Sachs has shown that if the tip of the primary radicle is cut off (and the tip will occasionally be gnawed off with seedlings in a state of nature) one of the secondary radicles grows perpendicularly downwards, in a manner which is analogous to the upward growth of a lateral shoot after the amputation of the leading shoot.  We have seen with radicles of the bean that if the primary radicle is merely compressed instead of being cut off, so that an excess of sap is directed into the secondary radicles, 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, it 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 seed-coats; but if the cotyledons are hypogean, it is the epicotyl which breaks forth.  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 as well as the ground, before any part of the stem protrudes; and then the petioles are almost invariably arched.