THE EPIDERMAL TISSUE. 



103 



le ovary {e.g. in Ricinus). They are most numerous where an active interchange 

 >f gases takes place between the plant and the external air ; for, considered physiologi- 

 cally, they are nothing but the mouths of the intercellular spaces of the inner tissue 



/hich open externally between the epidermal cells ; this is however always preceded 

 )y a peculiar development in a young cell of the epidermis. Since the stomata do not 

 irise till a late stage in the development of the internodes and leaves, or even after their 

 expansion, their arrangement is partially dependent on the already elongated form of the 

 epidermal cells ; if these are greatly elongated in one direction and arranged in rows 

 in Equisetum and the stem and leaves of many Monocotyledons and Pinus), the 



tomata are also arranged in longitudinal rows, the cleft lying in the direction of 

 khe axis of growth, the guard-cells right and left ; if the epidermal cells are irregular 

 )n a superficial view, curved, &c., the position of the stomata is more undefined and 

 ipparently irregular. The number of the stomata is generally extraordinarily great 

 the epidermis of organs containing chlorophyll. In 54 species A. Weiss counted on 

 me square mm. from r to 100 stomata, in 38 species 100-200, in 39 species 200-300, in 9 

 Species 400-500, and in 3 species 600-700 stomata. The origin of stomata is always 



le formation of a mother-cell, first of all by division of a young epidermal cell, which 

 sometimes preceded by several preparatory divisions in it ; this mother-cell becomes 



Fig. 85. — Development of tlie stomata in the leaf of Sedmn purpitrasceiis. A very young stomata ; B one ueai'ly 

 mature ; e e epidermal cells ; the numbers indicate the successive order of the preparatory divisions. 



then more and more rounded off, and the Guard-cells of the stoma are produced from 

 It by division. The variety of these processes up to the point when the cleft itself 

 ippears can hardly be explained in a few w^ords; I prefer therefore to describe some 

 examples more minutely. One of the simplest is afforded by the development of the 



tomata on the leaf of Hyadnthus orient alts, which has already been represented in 



vertical section in Figs. 61-64 (p. 77). The preparation for the formation of the stoma 



here very simple. A nearly cubical piece of a long epidermal cell is separated by 



septum, and this is the mother-cell of the stoma. It is divided by a longitudinal 



^all {i.e. by one parallel to the axis of growth of the leaf and at right angles to its 

 surface) into two equal cells, which round themselves off as they grow. How the 

 splitting of the partition-wall takes place has already been depicted in Figs, 61-64, and 

 ;an now easily be understood by the help of the surface-view in Figs. 84-86. In 



Iquisetum Umosum a similar appearance to that represented in Fig. 61 shows itself 

 immediately after the first formation of the mother-cells of the stomata; but the 



lother-cell undergoes in these cases three divisions, first one obliquely to the right, 



len one obliquely to the left, finally the middle cell is bisected by a wall at right 

 ingles to the surface. Four cells thus arise in one plane, of which the two outer ones 



row more rapidly, while the inner ones are forced downwards and beneath them; 



