150 THE LEAVES. 



Some adaptation is tlierefore needed, analogous to a self-acting valve, 

 which shall regulate the exhalation according to the supply. Such 

 an office is actually fulfilled by 



268. The Stomata, Stomates, or Breatliing-^ores (70). Through 

 the orifices which hear this name, exhalation principally takes place, 

 in all ordinary cases, where the epidermis is thick and firm enough 

 to prevent much escape of moisture by direct transudation. The 



stomata (Fig. 225 - 228) are always so 

 situated as to open directly into the hol- 

 low chambers, or air-cavities, which 

 pervade the parenchyma (Fig. 221), 

 especially the lower stratum, so as to 

 afford free communication between the 

 external air and the whole interior of 

 the leaf. The perforation of the epi- 

 dermis is between two (or rarely four) 

 delicate and commonly crescent-shaped cells, which, unlike the rest 

 of the epidermis, usually contain some chlorophyll, and in other re- 

 spects resemble the parenchyma beneath. When moistened these 

 guardian-cells change their form, becoming more crescentic as they 

 become more turgid, thereby separating in the middle and opening a 

 free communication between the outer air and the interior of the 

 leaf. As they become drier, they shorten and straighten, so as to 

 bring the sides of the two into contact and close the orifice.* The 

 use of this mechanism will be readily understood. So long as the leaf 



* They expand and contract most in the direction of their length ; and the 

 elongation and increased curvature when moist draws in the concave side and 

 so enlarges the aperture. The mechanism of the opening and shutting of sto- 

 mata has heen recently investigated by Mohl (in Bot. Zeitung for 1856, p. 697, 

 — an abstract of the memoir is given by C. P. Stone in Amer. Journal of Sci- 

 ence for March, 1857), — and these facts verified. Tire peculiar change of the 

 guardian-cells in form seems not entirely susceptible of mechanical explanation, 

 and is partly conti-oUed (like other vegetable movements) by the light of the 

 sun ; but it mainly depends upon endosmose. Mohl has clearly shown that, 

 while the guardian-cells themselves act so as to open the stomate in moisture 

 and close it in dryness, the adjacent cells of the epidermis in swelling when 

 moist tend to close the stomate, and their contraction when dry to open it ; — 

 so that the actual position at any time is a resultant of nicely adjusted opposing 

 forces. 



FIO. 225. A highly magnified piece of the epidermis of the Garden Balsam, with three 

 Btomata {after Brongniart). 



