Manner in which Hypocotyls, Epicotyls, etc., rise up and break through the ground.—­After the radicle has penetrated the ground and fixed the seed, the hypocotyls of all the dicotyledonous seedlings observed by us, which lift their cotyledons above the surface, break through the ground in the form of an arch.  When the cotyledons are hypogean, that is, remain buried in the soil, the hypocotyl is hardly developed, and the epicotyl or plumule rises in like manner as an arch through the ground.  In all, or at least in most of such cases, the downwardly bent apex remains for a time enclosed within the seed-coats.  With Corylus avellena the cotyledons are hypogean, and the epicotyl is arched; but in the particular case described in the last chapter its apex had been injured, and it grew laterally through the soil like a root; and in consequence of this it had emitted two secondary shoots, which likewise broke through the ground as arches.

Cyclamen does not produce any distinct stem, and only a single cotyledon appears at first;* its petiole

* This is the conclusion arrived at by Dr. H. Gressner (’Bot.  Zeitung,’ 1874, p. 837), who maintains that what has been considered by other botanists as the first true leaf is really the second cotyledon, which is greatly delayed in its development. [page 78]

breaks through the ground as an arch (Fig. 57).  Abronia has only a single fully developed cotyledon, but in this case it is the hypocotyl which first emerges and is arched.  Abronia umbellata, however, presents this peculiarity, that the enfolded blade of the one developed cotyledon (with the enclosed endosperm) whilst still beneath the surface has its apex upturned and parallel to the descending leg of the arched hypocotyl; but it is dragged out of the ground by the continued growth of the hypocotyl, with the apex pointing downward.  With Cycas pectinata the cotyledons are hypogean, and a true leaf first breaks through the ground with its petiole forming an arch.

Fig. 57.  Cyclamen Persicum:  seedling, figure enlarged:  c, blade of cotyledon, not yet expanded, with arched petiole beginning to straighten itself; h, hypocotyl developed into a corm; r, secondary radicles.

Fig. 58.  Acanthus mollis:  seedling with the hypogean cotyledon on the near side removed and the radicles cut off; a, blade of first leaf beginning to expand, with petiole still partially arched; b, second and opposite leaf, as yet very imperfectly developed; c, hypogean cotyledon on the opposite side.

In the genus Acanthus the cotyledons are likewise hypogean.  In A. mollis, a single leaf first breaks through the ground with its petiole arched, and with the opposite leaf much less developed, short, straight, of a yellowish colour, and with the petiole at first not half as thick as that of the other.  The undeveloped leaf is protected by standing beneath its arched fellow; and it is an instruc-[page 79] tive fact that it