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. We have met with only one exception, and that only a partial one, namely, with the petioles of the two first leaves of Acanthus candelabrum. With Delphinium nudicaule the petioles of the two cotyledons are com- [page 554] pletely confluent, and they break through the ground as an arch; afterwards the petioles of the successively formed early leaves are arched, and they are thus enabled to break through the base of the confluent petioles of the cotyledons. In the case of Megarrhiza, it is the plumule which breaks as an arch through the tube formed by the confluence of the cotyledon-petioles. With mature plants, the flower-stems and the leaves of some few species, and the rachis of several ferns, as they emerge separately from the ground, are likewise arched. The fact of so many different organs in plants of many kinds breaking through the ground under the form of an arch, shows that this must be in some manner highly important to them. According to Haberlandt, the tender growing apex is thus saved from abrasion, and this is probably the true explanation. But as both legs of the arch grow, their power of breaking through the ground will be much increased as long as the tip remains within the seed-coats and has a point of support. In the case of monocotyledons the plumule or cotyledon is rarely arched, as far as we have seen; but this is the case with the leaf-like cotyledon of the onion; and the crown of the arch is here strengthened by a special protuberance. In the Gramineae the summit of the straight, sheath-like cotyledon is developed into a hard sharp crest, which evidently serves for breaking through the earth. With dicotyledons the arching of the epicotyl or hypocotyl often appears as if it merely resulted from the manner in which the parts are packed within the seed; but it is doubtful whether this is the whole of the truth in any case, and it certainly was not so in several cases, in which the arching was seen to commence after the parts had wholly [page 555] escaped from the seed-coats. As the arching occurred in whatever position the seeds were placed, it is no doubt due to temporarily increased growth of the nature of epinasty or hyponasty along one side of the part.
As this habit of the hypocotyl to arch itself appears to be universal, it is probably of very ancient origin.