1909.] Vegetable Assimilation and Respiration. 23 



soaked with heavy rain, the amount of shrinkage in full sunlight was 

 astonishing. Even more svirprising was the sensitiveness of Sunflower leaves 

 to changes in the intensity of the sunlight. In the following pages is given 

 a selection of the results, which are of interest apart from the immediate 

 purpose for which they were obtained. 



Lengths were marked off on the leaves with fine black crosses over ai-eas carefully 

 selected for their fiatuess. They were measured with a millimetre scale : a support below 

 the leaf, consisting of a flat rule covei'ed with plush, yielded to the outstanding veins and 

 allowed the upper sui'face of the leaf to remain flat under the scale. 



Since the distances measured were of the order of 10 cm., a change of dimensions of 

 1 per cent, was easily detected. 



Measurements were taken both transversely and along the midrib. With a good flat 

 leaf four measurements were made (as in Leaves I and V, in the following tables and 

 diagrams), one across each half at its widest part, another right across the leaf nearer the 

 tip, and a fourth along the midrib. 



The percentage change of area was estimated by averaging the percentage changes in 

 the cross measurements, and adding the percentage longitudinal change. 



Most of the following observations were made on thi-ee plants growing in the University 

 Botanic Gaiden. Six leaves were chosen, and are referred to as I to VI respectively. 

 I, III, and IV were on the same plant and II on another plant in the same clump, in a 

 fairly exposed situation. V and VI were on a plant in the north border, sheltered from 

 the wind. 



The curves in fig. 3 represent the percentage changes which took place in 

 the area of Leaves I, II, and V over a period of seven days. The measure- 

 ments were begun on July 22, at 5 a.m., the dimensions obtained on this 

 occasion were taken as standards, and the variations subsequently observed 

 were calculated as percentages of those standard dimensions. The percentage 

 changes of area, estimated as described above, are plotted as ordinates against 

 the times as abscissse. 



Examining first the curves* for July 22, which was a bright sunny day 

 througliout, tlie remarkable fall in the curve for Leaf I, between 5 a.m. and 

 noon, is very striking, and indicates a shrinkage in area of 5 per cent. 

 Eecovery was rather less rapid, and by 3 p.m. the area was still 3'2 per cent, 

 less than at 5 a.m. The appearance of the leaf gave evident indications of a 

 decrease inturgidity by waving of the lamina, which was not so fully extended 

 as when quite turgid at 5 a.m. In addition to this the stalk curved 

 over, so that the leaf hung downwards. Unless signs of relative flaccidity 

 were being specially looked for, the position of the leaf might perhaps have 

 been attributed to heliotropic curvature, and the waving of the lamina to its 

 pendant position. In the early morning clear heliotropic movements were 

 noted. 



* It is scarcely necessary to remark that the curves themselves are only of rough 

 pictorial value, and do not accurately indicate the actual changes that take place in the 

 periods intervening between the actual measurements. 



