408 GROWTH OF PLANTS 



to 0.5 of a pH, but the juice is always acid, pH 5.9 being the highest recorded 

 after four days in 60 per cent CO2. Ethylene, which is a ripening hormone 

 produced by bananas during maturity and is also sometimes added to the 

 air to hasten ripening, did not modify the ascorbic acid content. Carbon 

 dioxide ^^ does not modify the ascorbic acid content of apples or green snap 

 beans but does reduce the ascorbic acid somewhat in freshly harvested 

 potatoes rich in this acid and in green pod peas, but has no effect on the 

 ascorbic acid in potatoes after a long period of storage. 



It is interesting to find CO 2, a naturally existing chemical environment 

 of plants, having such marked effects upon certain metabolic processes 

 here studied. It is likely that CO2 modifies many other metabolic processes 

 not included in these investigations. The effects produced by CO 2 are 

 strikingly like those produced by ethylene chlorhydrin which are discussed 

 in Chapter 7. 



Diurnal and Autumn Changes in Leaves of Deciduous Plants 



Diurnal changes. Denny "• i^. 20 j^ade a study of the diurnal changes 

 in leaves and examined critically the several methods that might be used 

 for accurately determining these changes. Offhand, so far as methods are 

 concerned, the problem looks simple. If one wants to find out what changes 

 occur in leaves during the night he can take a sample of leaves at dusk, a 

 similar sample at da^\^l, and analyze the two for the various organic con- 

 stituents making up the two samples. For the day changes, the first sample 

 could be taken at dsivm and the second one at dusk. The main problem is 

 to make sure that the two samples to be compared are comparable except 

 for the changes the 12 hours of darkness or light bring about. Denny's 

 work shows that getting comparable samples and a correct basis for calcu- 

 lation of the results are far from simple. He describes four different methods 

 that give reliable data, which, of course, give agreement in the changes 

 obtained by the four methods. 



He perfected the twin-leaf or, in compound leaves, twin-leaflet method. 

 His later studies with this method were made on mature leaves or leaflets 

 in order to avoid changes in weight involved in growth. In appljdng this 

 method, kinds of plants were selected in which the opposite leaves are of 

 approximately the same size. A sufficient number of the leaves are used 

 for each sample to give a low error due to possible]variation in size (25 to 

 300 per sample), and care was taken that on the average the two leaves of 

 the pair received equal light exposure. One leaf of each pair was used for 

 the first sample and the other was collected at a later hour for determining 

 the changes brought about by the day or night exposure. Since, as the data 

 show, the two samples were practically identical except for the changes 

 caused by the day or night exposure, the changes in each constituent are 

 reported in amount for the whole sample mthout reference to any measure- 

 ment that might vary with exposure, such as wet or dry weight, or area. 



