552 PHYSIOLOGY OF THE LEAYES OF CERTAIN CONIFEB3. 



to allude to the recent researches of Pringsheim*, which, if con- 

 firmed, will greatly modify our views as to the action of chloro- 

 phyll. These experiments go to show that the chlorophyll acts 

 as a screen, absorbing some of the luminous rays, and thus dimi- 

 nishing the intensity of the respiratory process, to which it serves 

 as a regulator. In this way the nutritive process and, in conse- 

 quence, growth are favoured at the expense of the function of 

 respiration f. 



It seems clear, then, that the differences in arrangement, rela- 

 tive position, form, organization, torsion, and power of elevation 

 and depression must be in direct relation to the functions which 

 the leaves have to perform. They are so many adaptations to the 

 work that has to be done, and are designed to promote the opera- 

 tions of the leaves as a whole, and to secure to each individual 

 leaf the greatest possible freedom of action consistent with the 

 least amount of encroachment on its neighbours. 



But although this is probably correct as a general statement, 

 the details require to be carefully worked out, as at present they 

 must be looked on more as indications of probabilities than as 

 substantiating theories. Thus, while the movements that have 

 been described in Abies Nordmanniana and other species are evi- 

 dently of some functional importance, it does not yet appear what 

 is their precise purpose, whether to increase respiratory activity, 

 to facilitate evaporation through the stomata, or to enhance the 

 functions of the chlorophyll. 



Pringsheim, " Untersuchungen iiber das Chlorophyll," Monatsb. der kcin. 



Akad. Wiss. Berlin, July 1879. 



t Duchartre ■ Element/ ed. 2, p. 427, notes that in the leaves of [some spe- 

 cies of] Narcissus and Dianthus the palisade cells are on both sides of the leaf. 



In these plants the leaves ascend, so that the two surfaces are equally exposed to 



the light, or nearly so, In the Hyacinth there are no palisade cells at all, 



each surface being here also nearly equally exposed, owing to the direction of 



the leaves ; and the same holds good, as I have lately observed, in the leaves 



of Tardanthus chinensis % 



