ACID ACCUMULATION IN SUCCULENTS 269 



content of the outer parts is higher (while that of the inner is 

 lower) in the morning than in the preceding evening. 



Obviously this movement must be taken into consideration in 

 a study of acidity. Since the dissolved acids of the interior sap 

 will be carried along in the stream, they will tend to accumulate 

 in the outer parts and thus increase their total acid content. The 

 main features of the conducting system of the sahuaro and bis- 

 naga are a central, woody, fibrovascular cylinder, with ramifica- 

 tions extending from it to the chlorophyllous tissue. 



With such an extensive conducting system changes in the 

 acidity of different zones due to movement of fluids might be 

 expected. However as closer consideration and a study of the 

 last tables will show, this effect is not marked. The amount of 

 acid carried to the outer parts by conduction from the inner must 

 be in proportion to the water, and the net result might be an in- 

 crease or decrease in the acidity per cc. of the outer sap, accord- 

 ing as the sap of the interior had a higher or lower concentration 

 than that of the exterior. However the increase of water con- 

 tent in a given area by movement from neighboring interior tissue 

 is only 1 to 3 per cent (see tables on a.m. and p.m. water dis- 

 tribution) , while the morning acidity may exceed that of the pre- 

 ceding afternoon by as much as 50 per cent. It would seem 

 therefore that sap movements play a very small part in the case 

 in question and we may safely assume that the observed effects 

 are due to nightly accumulation and daily destruction of the acid 

 products of carbohydrate metabolism. As a glance at the tables 

 will show, diurnal differences in the density of the sap are so slight 

 as to be negligible. 



One point to consider further is the relatively high concentra- 

 tion of acid observed in the outer parts in the early morning 

 analysis. The acidity near the surface is often double or even 

 triple the acidity of the sap at the center and usually becomes 

 progressively lower as the center is approached. While no 

 estimations of sugar concentration were made, it seems probable 

 that this higher acidity in the outer parts is due to a higher con- 

 centration of carbohydrates in the same regions. The synthesis 

 of carbohydrates can take place only in the green tissue of the 



