Rudimentary Cotyledons.—­A few remarks on this subject may be here interpolated.  It is well known that some dicotyledonous plants produce only a single cotyledon; for instance, certain species of Ranunculus, Corydalis, Chaerophyllum; and we will here endeavour to show that the loss of one or both cotyledons is apparently due to a store of nutriment being laid up in some other part, as in the hypocotyl or one of the two cotyledons, or one of the secondary radicles.

Fig. 60.  Citrus aurantium:  two young seedlings:  c, larger cotyledon; c’, smaller cotyledon; h, thickened hypocotyl; r, radicle.  In A the epicotyl is still arched, in B it has become erect. [page 95]

With the orange (Citrus aurantium) the cotyledons are hypogean, and one is larger than the other, as may be seen in A (Fig. 60).  In B the inequality is rather greater, and the stem has grown between the points of insertion of the two petioles, so that they do not stand opposite to one another; in another case the separation amounted to one-fifth of an inch.  The smaller cotyledon of one seedling was extremely thin, and not half the length of the larger one, so that it was clearly becoming rudimentary,* In all these seedlings the hypocotyl was enlarged or swollen.

Fig. 61.  Abronia umbellata:  seedling twice natural size:  c cotyledon; c’, rudimentary cotyledon; h, enlarged hypocotyl, with a heel or projection (h’) at the lower end; r, radicle.

With Abronia umbellata one of the cotyledons is quite rudimentary, as may be seen (c’) in Fig. 61.  In this specimen it consisted of a little green flap, 1/84th inch in length, destitute of a petiole and covered with glands like those on the fully developed cotyledon (c).  At first it stood opposite to the larger cotyledon; but as the petiole of the latter increased in length and grew in the same line with the hypocotyl (h), the rudiment appeared in older seedlings as if seated some way down the hypocotyl.  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.