BOOKS AND CURRENT LITERATURE 205 



cell walls and lead to permanent enlargement, without preliminary 

 changes in the walls themselves. (2) The turgor may remain unal- 

 tered but the extensibility of the walls may increase. (3) The turgor 

 of the enlarging cells may remain constant while that of the opposing 

 cells near the opposite surface of the leaf may decrease. The second 

 possibility is dismissed by Brown as improbable, but it should be re- 

 marked that changes in cell walls have not received much attention and 

 that such changes may be more important than they have appeared 

 to be. The author concludes that his first possibility is the most 

 probable, stating that the third alone cannot explain an adequate 

 water movement into the expanding cells from those with lowered 

 turgor. If the elastic cell walls of the enlarging tissue are stretched by 

 increased turgor and if the metabolic processes that render the enlarge- 

 ment permanent become active only somewhat later, it might be ex- 

 pected that leaves killed just after closure would reopen if the water in 

 them were replaced by some liquid in which the osmotically active 

 material is insoluble. Following this suggestion Brown killed leaves 

 in boiling water, transferred them to alcohol and then to xylene and 

 found that they reopened. Since sugars are practically insoluble in 

 xylene he concludes that they are of prime importance in producing 

 the increased osmotic activity of these cells when closure occurs. 

 Leaves that had been closed for some time, during which the perma- 

 nent alterations of the cell walls may be supposed to have occurred, 

 did not reopen in xylene. 



This rather remarkable experiment, which was also performed suc- 

 cessfully on Mimosa, should stimulate a more exact inquiry into the 

 details of its interpretation. Since cane sugar and glucose are only 

 slightlj' soluble in cold alcohol of sufficiently high concentration for 

 this experiment, the leaves ought to open in alcohol if these are the 

 sugars concerned. Maltose is more soluble in alcohol, however, and 

 might act as Brown supposes. The question of the source of the 

 sugar producing the increased concentration is also of interest, if we 

 assume with Brown that an increased sugar content is the cause of the 

 expansion of the cells near the ventral surface of the Dioncea leaf. The 

 author states that starch is never found in these cells of irritable leaves 

 and that some time after closure it becomes very abundant in the 

 cells of the dorsal portion. 



The location and nature of the membranes that are differentially 

 active after the death of the tissues in this xylene experiment are of 

 great general interest. If the protoplasmic membrane may thus func- 



