SUMMARY AND CONCLUSIONS. 



It is obvious that where so many variables are concerned as are presented in 

 the problems of the acidity and respiration of the cacti it becomes a difficult 

 matter properly to evaluate the results obtained. It will be well to summarize 

 briefly the main points which have been brought out before attempting any 

 general discussion. 



In all previous work upon the succulents, the cactus forms have not been 

 utilized as much as other types, which, indeed, is not remarkable considering 

 that no adequate supply of material could have been easily available. Had not 

 de Saussure's original observations on the anomalous gas interchange of fleshy 

 plants been made upon an Opuntia they might not have been used at all. 



Regarding these problems of acidity and gas interchange the following cate- 

 gorical statements may be made, some of which cover facts previously known. 



1. With rising acidity the total amount of acid in the tissues increases more 

 rapidly than does the concentration of the juice. The carbon dioxide produc- 

 tion with rising acidity is relatively small, while the oxygen absorption is large; 

 hence the C02/0a ratio is low. 



2. Light is the most important factor in the diurnal decrease of acidity, but 

 is less effective at low temperatures than at high ones. At higher temperatures 

 carbon dioxide is given off from the plant even in direct sunlight, as also is 

 oxygen. In diffuse light, carbon dioxide is produced in larger quantities and 

 oxygen is still absorbed. Dry joints absorb oxygen even in sunlight. 



3. Rising temperature, especially above 30 C., causes a decrease in acidity 

 when the latter is already high, but does not wholly inhibit the accumulation 

 of acid when near or at its minimum. With an increased evolution of carbon 

 dioxide due to rising temperature the C0 2 /0 2 ratio rises, since a corresponding 

 amount of oxygen is not absorbed. There is a lag in the response of carbon 

 dioxide evolution with both rising and falling temperatures. 



4. Acidity declines when the oxygen-supply is much above the normal and 

 tends to rise to a certain extent when the plant is deprived of that gas, provided 

 that the temperature is not too high. In an excess of oxygen the CO2/02 ratio 

 rises; in its absence the ratio remains stationary or decreases. 



5. Wounding causes a noticeable decrease in acidity, the amount being 

 proportional to the extent of the injury. Like other cases of decreasing 

 acidity, this implies an increased evolution of carbon dioxide and a consequent 

 raising of the gas-interchange ratio. The maximum evolution of carbon 

 dioxide is attained 24 hours after injury, slowly decreasing to the fourth day, 

 when it is again normal. The traumatic effect of the mere removal of a joint 

 from the plant is not sufficient to constitute an error for experimental purposes. 



6. Except in very young tissue which has the maximum acidity, the acid- 

 content and concentration do not vary greatly with age. The slightly lower 

 total acidity in 3-year-old turgid joints is due probably to the increase in the 

 thick-walled prosencyhmatic tissues rather than to actual decline in the acid 

 content of the parenchymatic cortex, since the concentration of the juice is 

 practically the same in the 1-year old tissue. The flaccid joints have a 

 generally lower acidity. The actual rate of the production of carbon dioxide 

 is greatest in the youngest tissues, next in the old turgid joints, and least of all 



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