176 



PLANT 



fastened by screws to the uprights ( ss ) holds it securely in 

 the ring." By appropriate modifications of this apparatus, 

 growth in diameter as well as in length can be investigated, 

 in stems as well as in roots. 



The following table indicates the effective pressures de- 

 veloped by growing roots : 



LONGITUDINAL PRESSURE 



TRANSVERSE PRESSURE 



These figures, obtained by averaging those in Tables I and 

 II of Pfeffer's paper, indicate that although the growing 

 parts are composed of such soft materials, they are capable 

 of developing under resistance a force which makes con- 

 tinued growth possible under all ordinary conditions. Cross 

 and longitudinal pressures developed by stems are probably 

 equal to those developed by roots, but the evident difficul- 

 ties in the way of measurements as exact as those for roots 

 cause the figures reported by Pfeffer to be somewhat lower, 

 as for example, 5.8 and 5.5 atmospheres for the longitu- 

 dinal and cross pressures developed by the stems of seed- 

 lings of Faba vulgaris. 



The growth of plants and animals is as a rule so slow 

 that accurate measurements are difficult to obtain. Besides 

 this, their behavior in other ways is very likely to compli- 

 cate any attempt to determine either the amount or the 

 rate of growth. The almost constant movement of plant- 

 parts out of doors under the influences of wind, sunlight, 

 warmth, etc., and the constant spontaneous movements 



