1909.] Vegetahle Assimilation and Respiration. 



29 



continuous observations still greater sensitiveness and rapidity of reaction to 

 cliano;es of illumination would have been revealed. 



It may be pointed out that the two halves of a leaf are not symmetrical even with 

 respect to their change of dimensions with change of conditions. For example, between 

 10 and 10.50 a.m. the width of the left half of the leaf had shrunk 4'2 per cent., while 

 the width of the right half at the same level had only decreased by 3"3 per cent. 

 Similar ditfei-ences were observed while making the measurements from which the curves 

 on fig. 3 were constructed. It was also noticed that the leaf shrinks more and sooner, 

 and recovers more tardily, towards the tip than it does nearer the base, a fact which is 

 doubtless to be explained by relative distance from the water supply. As was to be 

 expected from the nature of the tissues involved, the variations along the midrib are 

 comparatively small, but, as shown also on fig. 6, correspond closely in sign with the 

 variations in transverse dimensions. One other point is worthy of note. The right half 

 of Leaf I showed a faster rate of growth from day to day than did the left half, as well 

 as, if anything, a slightly smaller amount of shrinkage. With the possibility established 

 of such differences as these between two sides of a leaf, a certain degree of asymmetry 

 in the dry weight of the mesophyll itself is no longer surprising, although it does not 

 necessarily follow that true growth expansion involves an altei'atiou in dry weight per 

 unit of area luider comparable conditions of turgidity. 



Fig. 7 shows the results of observations on Leaves 7, 8, and 9, belonging to plants 

 growing on the .south side of the L^jjiversity Botany School. The curve for Leaf I on the 



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Fig. 7. — Shrinkage in area and recovery of leaves of Helianthus annuus on July 22, 1908. 

 7, 8, and 9, behind Botany School. I in open bed in Botanic Garden, for 

 comparison. Leaves 8 and 9 were in the shade of lime trees till 9 A.i:. ; Leaf 7 till 

 11 A.M. 



