238 



THE RATE OF GROWTH 



[CH. 



The delicate measuring apparatus now used shews sundry minor 

 but beautiful phenomena. A daily periodicity of growth is a 

 common thing* (Fig. 73). In the tree-cactuses the trunk expands 

 by day and shrinks again after nightfall ; for the stomata close in sun- 

 light, and transpiration is checked until the sun goes down. But 

 it is more usual for the trunk to shrink from sunrise until evening 

 and to swell from sunset until dawn ; for by dayhght the leaves lose 



70° F 



Black Poplar 



feet above ground, 59 inches 



July 24 



26 



Fig. 73. Growth of black poplar, shewing daily periodicity. 

 After A. Mallock. 



water faster, and in the dark they lose it slower, than the roots 

 replace it. The rapid midday loss of water even at the top of a 

 tail Sequoia is quickly followed by a measurable constriction of the 

 trunk fifty or even a hundred yards below f. 



* The diurnal periodicity is beautifully shewn in the case of the hop by Johannes 

 Schmidt, C.R. du Laboratoire Carlsberg, x, pp. 235-248, Copenhagen, 1913. 



•f This rapid movement is accounted for by Dixon and Joly's "cohesion-theory" 

 of the ascent of sap. The leaves shew innumerable minute menisci, or cup-shaped 

 water-surfaces, in their intercellular air-spaces. As water evaporates from these 

 the little cups deepen, capillarity increases its pull, and suffices to put in motion 

 the strands or columns of water which run continuously through the vessels of 

 wood, and withstand rupture even under a pull of 100-200 atmospheres. See 

 {int. al.) H. H. Dixon and J. Joly, On the ascent of sap, Phil. Trans. (B), clxxxvi, 

 p. 563, 1895; also Dixon's Transpiration and the Ascent of Sap, 8vo, London, 1914. 



