With Abronia arenaria there is a similar rudiment, which in one
* In Pachira aquatica, as described by Mr. R. I. Lynch ('Journal Linn. Soc. Bot.' vol. xvii. 1878, p. 147), one of the hypogean cotyledons is of immense size; the other is small and soon falls off; the pair do not always stand opposite. In another and very different water-plant, 'Trapa natans', one of the cotyledons, filled with farinaceous matter, is much larger than the other, which is scarcely visible, as is stated by Aug. de Candolle, 'Physiologie Veg.' tom. ii. p. 834, 1832. [page 96]
specimen was only 1/100th and in another 1/60th inch in length; it ultimately appeared as if seated halfway down the hypocotyl. In both these species the hypocotyl is so much enlarged, especially at a very early age, that it might almost be called a corm. The lower end forms a heel or projection, the use of which will hereafter be described.
In Cyclamen Persicum the hypocotyl, even whilst still within the seed, is enlarged into a regular corm,* and only a single cotyledon is at first developed (see former Fig. 57). With Ranunculus ficaria two cotyledons are never produced, and here one of the secondary radicles is developed at an early age into a so-called bulb.** Again, certain species of Chaerophyllum and Corydalis produce only a single cotyledon;*** in the former the hypocotyl, and in the latter the radicle is enlarged, according to Irmisch, into a bulb.
In the several foregoing cases one of the cotyledons is delayed in its development, or reduced in size, or rendered rudimentary, or quite aborted; but in other cases both cotyledons are represented by mere rudiments. With Opuntia basilaris this is not the case, for both cotyledons are thick and large, and the hypocotyl shows at first no signs of enlargement; but afterwards, when the cotyledons have withered and disarticulated themselves, it becomes thickened, and from its tapering form, together with its smooth, tough, brown skin, appears, when ultimately drawn down to some depth into the soil, like a root. On the other
* Dr. H. Gressner, 'Bot. Zeitung,' 1874, p. 824.
** Irmisch, 'Beiträge zur Morphologie der Pflanzen,' 1854, pp. 11, 12; 'Bot. Zeitung,' 1874, p. 805.
*** Delpino, 'Rivista Botanica,' 1877, p. 21. It is evident from Vaucher's account ('Hist. Phys. des Plantes d'Europe,' tom. i. 1841, p. 149) of the germination of the seeds of several species of Corydalis, that the bulb or tubercule begins to be formed at an extremely early age. [page 97]
hand, with several other Cacteae, the hypocotyl is from the first much enlarged, and both cotyledons are almost or quite rudimentary. Thus with Cereus Landbeckii two little triangular projections, representing the cotyledons, are narrower than the hypocotyl, which is pear-shaped, with the point downwards. In Rhipsalis cassytha the cotyledons are represented by mere points on the enlarged hypocotyl. In Echinocactus viridescens the hypocotyl is globular, with two little prominences on its summit. In Pilocereus Houlletii the hypocotyl, much swollen in the upper part, is merely notched on the summit; and each side of the notch evidently represents a cotyledon. Stapelia sarpedon, a member of the very distinct family of the Asclepiadeae, is fleshy like a cactus; and here again the upper part of the flattened hypocotyl is much thickened and bears two minute cotyledons, which, measured internally, were only .15 inch in length, and in breadth not equal to one-fourth of the diameter of the hypocotyl in its narrow axis; yet these minute cotyledons are probably not quite useless, for when the hypocotyl breaks through the ground in the form of an arch, they are closed or pressed against one another, and thus protect the plumule. They afterwards open.
From the several cases now given, which refer to widely distinct plants, we may infer that there is some close connection between the reduced size of one or both cotyledons and the formation, by the enlargement of the hypocotyl or of the radicle, of a so-called bulb.