I0I4 



A TEXTBOOK OF THEORETICAL BOTANY 



is, in fact, closely correlated with the direction of the underlying veins in the 

 mesophyll. Here again differential growth rates are probably responsible. 

 More rapid stretching of the leaf tissue along the veins is shown by the more 

 elongated epidermal cells usually formed above them and this may account 

 for the elongation of stomata in the same general direction. 



The linear leaves of many Monocotyledons, in which growth is continuous 

 from the leaf base, usually have the epidermal cells and the stomata in longi- 

 tudinal rows (Fig. 1002), the latter also having their long axes regularly in 



P"'I#'M^-'^*:' 



o. 



1002.- 

 n arrow 

 longitU' 



Iris pallida. Epidermis of leaf showing 

 elongated cells and stomata arranged 

 dinallv. 



the same direction. ^Monocotyledons, such as Arum, with net venation, have 

 randomized stomata like those of Dicotyledons. Transversely orientated 

 stomata are uncommon and are nearly always associated with reduced leaves 

 and xeromorphic structure. 



The differentiation of stomatal mother-cells from epidermal cells begins 

 at a very early stage of leaf development. It involves that rare occurence, 

 the division of a cell into two markedly unequal portions, a large cell which 

 becomes epidermal and a smaller cell, always on the side towards the apex, 

 which becomes a stomatal initial cell. 



The first differentiation of stomata is usually coincident with the beginning 

 of expansion in the leaf rudiment. It is not simultaneous all over the surface, 

 but begins near the leaf apex, the differentiation in other parts being concurrent 

 with the appearance of air spaces in the mesophyll. Stomata are formed 

 successively for some time, the earliest to appear showing an orientation 

 related to the direction of growth tensions, while those which appear later 

 are seemingly irregular in alignment. 



The stomatal initial cell is in most cases also the mother cell of the stoma, 

 but in certain families the initial cell mav divide once or several times, 



il 



