DR. M. J. SCHLEIDEN ON PHYTOGENESIS. 303 



This process is more easily discernible in articulated hairs, and 

 such as have a head consisting of several cells, where the same 

 appearances, which I have so frequently observed in the young 

 embryo, and such as Mirbel has so beautifully described in the 

 development of the gemmae in the cups of Marchantia, may be 

 easily and beautifully seen, for instance in the common potatoe. 

 Meyen also has published similar observations, although he still 

 expresses himself with some doubt. (Wiegmann's Archiv, 1837, 

 vol. ii. p. 22.) 



It is not until after as many cells are formed as the organ re- 

 quires for its completion that the walls of the cells become firmer ; 

 and then commences the development 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 of the 

 epidermis. At an early period a stripe of more transparent 

 cells is defined in the axis of the leaf which is in the act of 

 forming, in which no moi'e new cells are developed, and these 

 cells soon considerably exceed in size the cells of the remaining 

 mass, which are constantly becoming smaller by continual divi- 

 sion. These cells are the foundation of the future vascular bun- 

 dle which forms the midrib of the leaf. For Avhile the paren- 

 chymatous cells subsequently expand on all sides, these cells are 

 only developed in their longitudinal dimension, and are thus able, 

 although fewer in number, to follow the expansion of the other 

 cells in the longitudinal direction of the leaf. It is not tiU a later 

 period that these cells, by a difference of the internal depositions, 

 separate themselves into spiral vessels and cells of the Hber. 

 The spiral vessels begin to be visible in the newly-fonned parts, 

 and also in the entire bud, always in the immediate vicinity of old, 

 already formed spiral vessels ; and they proceed in this manner 

 away from the stem into the new parts. I do not under- 

 stand therefore what is meant when the fibres of the stem are 

 regarded as proceeding from the buds ; one might quite as well 

 consider the river as running from the ocean to its source. 



A similar process takes place in the development of the side 

 nerves of leaves. The formation of new cells generally ceases 

 quite early in the outermost layer of cells. The cells are soon 

 filled with a hmpid fluid, and naturally become, on the expansion 

 of the subjacent parenchyma, superficial, flat, and expanded. 



The cells of the vascular bundle and of the epidermis appear 

 iu this way to be less potentialized [minder pot emir f\ — are as 



