104 SIZES OF STOMATA [CH. 



substances (i.e. substances with a powerful attraction for 

 water) such as carbo-hydrates, and that the tension thus 

 set up brings this very mechanism into play. 



On the other hand, regarding the guard-cells for a 

 moment as rigid, an osmotic influx of water into the 

 subsidiary cells would tend to close the stomata ; and 

 other complications exist according to the distribution of 

 the osmotic substances in these and other cells. 



Apparently no simple generalisation covers all the 

 facts, but we may say that in ordinary cases the stomata 

 tend to be closed at night and in damp weather, because 

 the epidermis-cells are then turgid with water and so drive 

 the guard-cells together : in bright sunlight, on the other 

 hand, when transpiration is at its maximum, these cells are 

 more flaccid and so draw the guard-cells apart, while the 

 latter enhance this effect by their own osmotic activity. 



There are many exceptions, however, to the rule that 

 the stomata are open in bright light, and it is abundantly 

 evident that internal as well as external stimuli may be 

 concerned. 



As regards the sizes of stomata they vary in different 

 plants. The averages estimated vary from 0*0002 to 

 0*0008 of a sq. mm. in area ; and from 0*02 to 0*08 mm. 

 long and 0*01 to 0*08 mm. broad, while the area of the 

 orifice has been put at 0*00047 to 0*0000137 sq. mm. 

 The aperture at its widest in the largest stomata is about 

 -^ mm., and in the smallest about T ^ mm., the slit being 

 about six times as long as broad. The largest occur in 

 Monocotyledons such as Orchids, Lilies, Grasses, &c., and 

 in Conifers : the smallest in Water Lilies, Olives, Figs, 

 Amaranthus, &c. 



Owing to their extremely small size it is obvious that 

 stomata may easily become blocked by dew- or rain-drops, 

 and many of the peculiarities of the epidermis are adapted 



