36 CELLULAR TISSUE. 



The size of the mature, full-grown stomata is usually smaller than the average 

 size of the adjoining epidermal cells, often extremely small in comparison with these, 

 e.g. Salvinia; on the same surface, e.g. the leaf-surface, it is in the majority of 

 cases on the whole uniform with slight variations. The absolute size of the 

 space which they occupy in the epidermal surface lies, according to the measure- 

 ments made by A. Weiss ^ on 150 plants, between o'oooii'"™n (Amarantus cau- 

 datus; length and breadth =o-oi6'"'") and ooo459™inn (Amaryllis formosissima, 

 length 0074, breadth 0-079'"™), i^i most cases between o.ooo2i»"ii n and o-ooo8iiin» D . 

 The size of the open slit apparently bears an almost constant relation to that of the 

 whole apparatus, but exact measurements of this have been made only for few cases. 



The size and form of the slit as well as of the guard-cells vary regularly in 

 the same stoma, according to the turgescence and tension of the membranes of 

 the guard-cells themselves and of the surrounding epidermis ; this turgescence and 

 tension depending upon the supply of water, and the effect of light and heat. 

 The curvature of the side of the guard-cells next the slit, and accordingly the 

 opening of the slit, may in each special case increase to a definite maximum, and 

 on the other hand diminish till the slit is completely and firmly closed. With 

 these changes of curvature changes in the general form of the guard-cells are 

 in each case connected. According to H. v. Mohl insolation and supply of water, 

 according to N. Miiller heat and supply of water, bring about the widening of the 

 slit^ The very large stomata of Lilium martagon, candidum and bulbiferum, 

 widen the slit, according to Mohl, to a breadth of Y,,o™ni to V,24™"^ on the "uninjured 

 leaf, at the margins of separated pieces of epidermis to Vsg™'^^ ; on the uninjured 

 leaf of Zea mais to Vi-k""" ; on the separated epidermis of Amaryllis formosissima 

 to V^g""'^. The slit remains meanwhile always at least six to seven times longer than 

 broad. Unger' quotes the size of the open slit in Agapanthus umbellatus at 

 0-000047'""! D, of Ajuga genevensis at o.ooooi37"""a . 



The water-pores to be described below (Sect. 8) assume much larger dimensions, 

 as also the stomata on the leaf of the Kaulfussias. The latter are visible to 

 the naked eye as round holes, which are surrounded moreover by a pair of 

 guard-cells apparently incapable of change of curvature. 



The absolute height of the guard-cells, after what has been already said, 

 requires no description. Compared with the epidermal cells, or the many-layered 

 epidermis of the same surface, the height of the guard-cells is usually insignificant, 

 often very small ; at most they are of equal height with them (e. g. Hyacinthus 

 orientalis ■*), Lilium candidum", Helleborus niger. Fuchsia'' (Fig. 10). The position 

 of stomata relatively to the outer surface of the epidermis is closely connected with 

 these differences. When the height of the guard-cells is equal to that of the 

 epidermal cells the outer surfaces of both lie approximately in the same plane. The 

 same occurs in a series of cases where the height is unequal ; here the respiratory 



' Pringsheim's Jahrb, IV. 



^ Compare on the mechanism, which must not here be discussed, and which is not even yet 

 fully explained, the fundamental work of Mohl, Botan. Zeitg. 1856, p. 697; Sachs, vol. IV of this 

 Handbook, p. 255 ; N. Miiller, in Pringsh Jahrb. \'III. p. 75. [Schwendener, / r.] 



' Anat. und Physiol, p. 334. * Strasburger, I.e. fig. 14. 



^ Mohl, I.e. fig. 6. * Unger, Anat. und Physiol, p. 190. 



