272 ROUND THE YEAR 



for a moment to consider what is the special use of the 

 cushion to the active leaf. 



It is an organ of movement. By means of the 

 cushion the leaf changes its attitude, inclines its 

 surface to catch the light, droops at night, and in some 

 cases droops when touched. The delicate adjustments 

 by which the leaf sets itself in the best position both 

 with respect to light or neighbouring leaves are 

 effected by the cushion. The mechanism of adjust- 

 ment depends upon the turgidity (distention by water) 

 of the cellular cushion. The cells can either absorb 

 water from neighbouring tissues, or give it out, and 

 swelling or contraction follows. There may be 

 swelling on one side and contraction on the other ; 

 the swelling may be either temporary or permanent. 

 Swelling on one side causes a leaf-stalk or young stem 

 to incline to the opposite side. Sometimes the cellular 

 tissues of a shoot swell on every side in succession. 

 Then the shoot sweeps round and round in a regular 

 nutation, bowing to every point of the compass in the 

 course of its revolution. 



The cushion plays an important part in the fall of 

 the leaf. Here the block takes place, which cuts off 

 the supply of water passing upwards from the stem 

 and roots to the leaf. Here too the separating 

 layer forms, which at length severs the leaf from the 

 branch. 



The separating layer ends by producing a transverse 

 cut through all the cellular tissues of the leaf-stalk, 

 sparing only the vessels and fibres, though these too 

 it will ultimately break through. The parting of the 

 cellular tissues greatly increases the ease with which 



