With germinating monocotyledonous seeds, of which, however, we did not observe a large number, the plumules, for instance, those of Asparagus and Canna, are straight whilst breaking through the ground.  With the Gramineae, the sheath-like cotyledons are likewise straight; they, however, terminate in a sharp crest, which is white and somewhat indurated; and this structure obviously facilitates their emergence from the soil:  the first true leaves escape from the sheath through a slit beneath the chisel-like apex and at right angles to it.  In the case of the onion (Allium cepa) we again meet with an arch; the leaf-like cotyledon being abruptly bowed, when it breaks through the ground, with the apex still enclosed within the seed-coats.  The crown of the arch, as previously described, is developed into a white conical protuberance, which we may safely believe to be a special adaptation for this office.

The fact of so many organs of different kinds—­hypocotyls and epicotyls, the petioles of some cotyledons and of some first leaves, the cotyledons of the onion, the rachis of some ferns, and some flower-stems—­being all arched whilst they break through the ground, shows how just are Dr. Haberlandt’s* remarks on the importance of the arch to seedling plants.  He attributes its chief importance to the upper, young, and more tender parts of the hypocotyl

* ‘Die Schutzeinrichtungen in der Entwickelung der Keimpflanze,’ 1877.  We have learned much from this interesting essay, though our observations lead us to differ on some points from the author. [page 88]

or epicotyl, being thus saved from abrasion and pressure whilst breaking through the ground.  But we think that some importance may be attributed to the increased force gained by the hypocotyl, epicotyl, or other organ by being at first arched; for both legs of the arch increase in length, and both have points of resistance as long as the tip remains enclosed within the seed-coats; and thus the crown of the arch is pushed up through the earth with twice as much force as that which a straight hypocotyl, etc., could exert.  As soon, however, as the upper end has freed itself, all the work has to be done by the basal leg.  In the case of the epicotyl of the common bean, the basal leg (the apex having freed itself from the seed-coats) grew upwards with a force sufficient to lift a thin plate of zinc, loaded with 12 ounces.  Two more ounces were added, and the 14 ounces were lifted up to a very little height, and then the epicotyl yielded and bent to one side.

With respect to the primary cause of the arching process, we long thought in the case of many seedlings that this might be attributed to the manner in which the hypocotyl or epicotyl was packed and curved within the seed-coats; and that the arched shape thus acquired was merely retained until the parts in question reached the surface of the ground.  But it is doubtful whether this is the whole of the truth in any case.