TURGESCENCE. 57 



of the tube in which the stem is inserted is filled with water and the water is 

 allowed to partly fill the other arm. Into this other arm is then poured 

 mercury. The greater weight of the mercury causes such pressure upon the 

 water that it is pushed into the stem, where it passes up through the vessels 

 in the stems and leaves, and is brought more quickly and surely to the cells 

 which contain the protoplasm and cell-sap, so that turgidity is more quickly 

 and certainly attained. 



114. Tissue tensions. Besides the turgescence of the cells of 

 the leaves and shoots there are certain tissue tensions without 

 which certain tender and succulent shoots, etc., would be limp, 

 and would droop. There are a number of plants usually accessi- 

 ble, some at one season and some at others, which may be used 

 to illustrate tissue tension. 



115. Longitudinal tissue tension. For this in early summer 

 one may use the young and succulent shoots of the elder 

 (sambucus); or the petioles of rhubarb during the summer and 

 early autumn ; or the petioles of richardia. Petioles of cala- 

 dium are excellent for this purpose, and these may be had at 

 almost any season of the year from the greenhouses, and are 

 thus especially advantageous for work during late autumn or 

 winter. The tension is so strong that a portion of such a 

 petiole T.oi$cm long is ample to demonstrate it. As we grasp 

 the lower end of the petiole of a caladium, or rhubarb leaf, we 

 observe how rigid it is, and how well it supports the heavy 

 expanded lamina of the leaf. 



116. The ends of a portion of such a petiole or other object 

 which may be used are cut off squarely. With a knife a strip 

 from 2-ynm in thickness is removed from one side the full 

 length of the object. This strip we now find is shorter than 

 the larger part from which it was removed. The outer tissue 

 then exerts a tension upon the petiole which tends to shorten 

 it. Let us remove another strip lying next this one, and 

 another, and so on until the outer tissues remain only upon 

 one side. The object will now bend toward that side. Now 

 remove this strip and compare the length of the strips re- 

 moved with the central portion. We find that they are nmcb 



