PIIYTOGENESIS. 



_ : . > . > 



the young embryo, and sucli as Mirbel has so beautifully 

 described in the development of the gemma' in the cups of 

 Marchantia, may be readily and beautifully seen; for example, 

 in the common potato. Meyeu lias also made Bimilai ob- 

 servations, although he still expresses himself with some doubt 

 on the subject. (Wiegmann's Archiv, \K:)7, 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 libt r. 

 The spiral vessels are always first perceptible in the new In- 

 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 Bide 

 nerves of leaves. The formation of new cells generally ceac 

 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. 



