550 



AND 



lowered. M. J. Chatin (who first placed this fact on record*, 

 though it had been long familiar to growers of these plants) asserts 

 that in the middle of the day the plant (Abies Nordmanniand) 

 has a predominant green hue, but when the light is more 

 diffused, as in the evening or early morning, then the plant 

 assumes a milky-white appearance. This appearance is due to 

 the elevation of the glaucous under-surface of the leaves. The 

 movements in question are so obvious during the period of active 

 vegetation that no doubt can exist on the subject. Unlike 

 M. Chatin, however, I have observed that the white hue of Abies 

 JSordmanniana is more conspicuous when the branches are exposed 

 to the full rays of the sun. The same remark holds good in the 

 case of Picea sitchensis (Menziesii) , ajanensis, and bicolor, the 

 leaves of the first of which are more like those of the Spruces 

 than of the Silver Firs f. 



The functions of the leaves are, as is well known, threefold : 



a. Assimilation, the phenomena attendant upon which are 

 the formation of chlorophyll and other products, and the elimina- 

 tion through the stomates of oxygen. Exposure to light is requi- 

 site for this process. 



(3. Sespiration, or the appropriation of oxygen and the elimi- 

 nation of carbonic acid-gas, as in animals. For this process expo- 

 sure to light is not a direct essential. 



y. Exhalation of Watery Vapour. This is intensified by heat ; 

 but whether light per se has any influence upon it is open to 



question. 



The stomates are the principal means by which the gases or the 

 exudations make their exit. The layer of thick-walled epidermal 



cells, and the still thicker hypoderm-cells beneath, would naturally 



offer great hindrance either to the entrance or to the exit of 



gases or of water ; hence the stomates might fairly be considered 



the agents through whose means inhalation and exhalation take 



place, even had this not been proved by direct experiment. 



much confusion in the synonymy of these species, it is probable that the discre- 

 pancies may be accounted for by this circumstance. 



* "Sur les raouvements periodiques des feuilles d' Abies Nordmanniana. 1 

 Abstract in ■ Revue Bibliographique ' of the Bull. Soc. Bot. France, t. xxiii. p. 103 

 (1876). 



t In connexion with these movements it may be of interest to recall the ob- 

 servations made by me on the revolving nutation of the leader of Abies Aord* 

 manniana, see Gard. Chron. ix. 1878, pp. 247 & 826. 



