184 PHYSIOLOGICAL BOTANY. PART II. 



organs of plants, as that the spongioles are their real 

 absorbing organs. As the under surfaces of leaves are 

 in general more plentifully supplied with stomata than 

 their upper surfaces, the exhalation is there the most 

 abundant. Plants which live under water have no sto- 

 mata ; but as they have no true epidermis either, they 

 rapidly fade when exposed to the air, from the more de- 

 cided effects of evaporation alone. 



(170.) Stimulants to Exhalation. The manner in 

 which the stomata act is unknown ; and consequently 

 we are compelled to ascribe the function which they 

 perform to the immediate operation of the vital force. 

 The stimulus by which their activity is sustained, is 

 u.ainly if not entirely due to the influence of light ; for 

 the exhalation ceases when the plant is carried into a 

 darkened chamber, and is restored upon its return to 

 the light. Even lamplight is, to a certain extent, suf- 

 ficient for maintaining this action. The effects of ex- 

 halation are remarkably apparent about sunrise, when 

 the temperature is low, and the moisture exhaled is not 

 readily carried off; it then accumulates, and is deposited 

 in innumerable drops upon the surface and edges of the 

 leaves, and is generally mistaken for the effects of dew : 

 but as it collects equally on plants which are under shel- 

 ter as on those which are openly exposed, this cannot be 

 the true cause. It is by no means clear that an elevation 

 of temperature has any effect in modifying this func- 

 tion ; but since it undoubtedly increases the quantity of 

 the evaporation, it becomes difficult to decide whether 

 any portion of the result is due to an increased ex- 

 halation also. The manner in which the direct rays of 

 the sun act in stimulating this function, is well known 

 to those who are aware how necessary it is in order to 

 preserve the beauty and freshness of a nosegay, to keep it 

 constantly in the shade. There are certain succulent 

 plants which possess so few stomata that they may be 

 preserved out of the ground for many days and even 

 months, without perishing from want of moisture ; and 

 it will frequently happen that Sedums, and other plants 



