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THE POPULAR SCIENCE MONTHLY 



are able to hold their leaves and other organs in position. This could 

 not be done without the exertion of considerable pressure, for their 

 delicately constructed leaves and other organs often assume positions 

 requiring a great deal of support. In trees and shrubs there is a large 

 amount of mechanical tissue which supplies the necessary means for 

 supporting the various members. 



What is termed the " power of growth " can be determined by learn- 

 ing the amount of weight required to stretch a rapidly growing stem to 

 its original length after the turgidity of the cells has been destroyed by 

 placing them in plasmolyzing solutions, such as a 10 per cent, solution 

 of potassium nitrate. The mean area of a cross section of a stem in 

 millimeters, divided by the amount of weight obtained in grams, gives 

 the number of grams per square millimeter of surface, and, as previously 

 stated, there is usually obtained by this method a pressure of one to 

 three atmospheres or more in the cell for ordinary growth. 



Fig. 7. Method of determining radial pressure of growing roots. (After Pfeffer. ) 



While this osmotic pressure is common in ordinary growing organs, 

 it does not necessarily follow that it constitutes the limit, since in the 

 case of the ostrich fern previously referred to it was much higher. 

 When growth is mechanically restricted or the organism has obstacles 

 to overcome, the cell turgescence or osmotic pressure may be greatly 

 increased owing to the resulting stimulus, and this is what occurred in 

 the case of the ferns. 



If a cross section of a stem is made and the bark split vertically, a 

 noticeable shrinkage of the bark takes place, demonstrating a difference 

 in tension between the outer and inner tissues. On the other hand, if 

 longitudinal slices are taken from the outside of a common sunflower 

 stem, they will shorten from 1 to 4 per cent, of their length, while the 

 tissues from the center of the stem (pith) will lengthen from 1 to 6 

 per cent, when removed. It is clear from these observations that the 



