THEIR ANATOMY. 157 



the sun's rays, the greater the speed at which the vegetable ma- 

 chinery is driven. But whenever the 'supply at the root fails, the 

 foliage begins to flag and droop, as is so often seen under a sultry 

 meridian sun ; and if the exhaustion proceeds beyond a certain 

 point, the leaves inevitably wither and perish. Some adaptation is 

 therefore needed, analogous to the governor in machinery, or the 

 self-acting valve, which shall regulate the exhalation according to 

 the supply. Such an office is actually fulfilled by 



268. The Stomata, Stomates, or Breathing -pores (70). Through 

 the valvular orifices which bear 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 transu- 

 dation. The stomata (Fig. 181-183, 187) are situated so as to 

 open directly into the hollow chambers, or air-cavities, which per- 

 vade the parenchyma (Fig. 179, 186), especially the lower stra- 

 tum ; so as to afford free communication between the external air 

 and the whole interior of the leaf. The perforation of the epider- 

 mis is between two (or rarely four) small and delicate cells, which, 

 unlike the rest of the epidermis, usually contain some chloro- 

 phyll, and in other respects resemble the parenchyma beneath. 

 Their exact mechanism is not very well made out ; but it appears 

 that, when moist, these hygrometric cells become turgid, and in 

 elongating diverge or curve outwardly in their middle, where they 

 do not cohere, so as to open a free communication between the 

 outer air and the interior of the leaf. When dry, they incline to 

 shorten and straighten, so as to bring their sides into contact and 

 close the orifice completely. This structure is sufficiently illus- 

 trated in the figures referred to, and es- 

 pecially in those of the Lily, where the 

 stomata are unusually large and easy of 

 examination. The action and use of 

 this mechanism will readily be under- 

 stood. So long as the leaf is in a moist 

 atmosphere, and is freely supplied with 

 sap by the stem and roots, the cells that 

 guard the orifice are expanded, and the 

 open stomata allow the free escape of 

 moisture by evaporation. But when the supply fails, and the pa- 



FIG. 187. A highly magnified piece of the epidermis of the Garden Balsam, with three 

 stomata (after Brongniart). 



14 



