FORMATION OF THE COMMON WALL OF CELLS. 



S 



Hyacinthus orientalis as an example. Figs. 61-64 are transverse sections perpendicular 

 to the upper surface of the leaf; ee in all of them are the epidermis-cells, pp the paren- 

 chyma of the leaf. The stoma (5) is formed of a smaller epidermis-cell which divides 



Figs. 61-63.— Development of tlic stoniata of the leaf o{ HjncuiiAus oj-ientalis, seen in transverse section (x8oo). 



FIG. 64. 



into two equal sister-cells by a wall standing 

 vertically to the surface of the leaf; in Fig. 

 61, 5, this has just taken place; the partition- 

 wall is formed ^ it appears as a very thin 

 simple lamella, which soon attains greater 

 thickness, and especially thickens more rapidly 

 where it meets at right angles the wall of the 

 mother-cell without and within (Fig, 62, ^). 

 The thickening mass appears at first quite ho- 

 mogeneous; afterwards an indication of stratifi- 

 cation is to be observed, and the first trace of 

 a separation of the still simple lamella into two 

 lamellae (Fig. 62, B). In Fig. 63, /, the split- 

 ting is already completed ; the growth of the separated lamellae now proceeds in a 

 peculiar manner, so that a cleft arises narrower in the middle, wider without and 

 within, which unites the intercellular space i (the air-cavity) with the external air 

 (Fig. 64). It is worth mention that before the division of the mother-cell, an obvious 

 not very thin cuticle has already overspread it together with the adjoining cells of 

 the epidermis. This is especially to be recognised in the condition B, Fig. 62, while still 

 continuous; by the splitting of the partition-wall into two lamellae it finally becomes 

 ruptured (Fig. 63); and by the cuticularising of the outermost layer of the now 

 separated lamellae the cuticle is afterwards continued over the surfaces of the cleft 

 (Fig. 64). If the process of the formation of the stoma is followed up on a superficial 

 view, it shows that the splitting of the partition-wall does not extend through its whole 

 surface, but that a portion still remains above and below (taking the leaf in a vertical 

 position) as a simple lamella (cf. sect. 15, Figs. 73-75). Both the Guard-cells (the cells 

 which enclose the cleft) are not only distinguished from the other epidermis-cells by 

 this peculiar mode of division and of growth ; they also differ from them by containing 

 chlorophyll and starch. 



(2) In the family of Marchantieae belonging to the Hepaticae, the origin and struc- 

 ture of the stomata (Fig. 65, B, sp) is much more complicated ; of this we must speak 

 hereafter. Here it need only be pointed out that even before their formation the epi- 

 dermis-cells have become detached from those lying beneath ; and in such a manner 

 that the separating surfaces (seen from above) represent rhomboidal plates beneath the 

 epidermis, which are marked off from one another by the walls of unseparated cells 



^ I was unable to detect nuclei immediately before and for a consideiable time after the 

 division. 



