PiTYTOGENESIS. 255 
the young embryo, and such as Mirbel has so_ beautifully 
described in the development of the gemmz in the cups of 
Marchantia, may be readily and beautifully seen; for example, 
in the common potato. Meyen has also made similar ob- 
servations, 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 deve- 
loped, 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 direc- 
tion, 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 conse- 
quence of a difference in the depositions in their interior, be- 
come 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. 
