PHYTOGENESIS. 255
the young embryo, and such as Mirbel has so beautifully
described in the development of the gemmæ in the cups of
Marchantia, may be readily and beautifully seen; for example,
in the common potato. Meyen has also made similar observations, although he still expresses himself with some doubt on the subject. (Wiegmann’s Archiv, 1837, vol. ii, p. 22.)
It is not until after as many cells are formed as the organ requires for its completion that the cell-walls become firmer, and then commences the unfolding of the organ by the mere expansion of the cells already formed.
But I must here enter somewhat more into detail, in order to explain the probable origin of the vascular bundles and epidermis. At a somewhat early period a stripe of more trans- parent cells is defined in the axis of the leaf which is in the act of formation, within which no more new ones are developed, and these cells soon considerably exceed in size those of the remaining mass, which are constantly becoming smaller by continual division. These cells are the basis of the future vascular bundle which forms the midrib of the leaf; for whilst the parenchymatous cells subsequently expand in every direction, these are developed in their longitudinal dimension only, and are thus enabled, although fewer in number, to follow the expansion of the other cells in the longitudinal direction of the leaf. It is not till a later period that these cells, in consequence of a difference in the depositions in their interior, become distinguished into spiral vessels and cells of the liber. The spiral vessels are always first perceptible in the newly-formed parts, and in the entire bud also, in the immediate neighbourhood of the old, previously-formed spiral vessels; and they proceed in this manner downwards from the stem into the new parts. I do not understand therefore what is intended when the fibres of the stem are regarded as descending from the buds; one might just as well conceive the river to run from the ocean to its source.
A similar process occurs in the development of the side nerves of leaves. The formation of new cells generally ceases at an early period in the outermost layers of cells. The cells there are soon filled with a limpid fluid, and, by the expansion of the subjacent parenchyma, naturally become superficial, flat, and expanded.
