THE PLANT AND ITS ENVIRONMENT 1G3 



themselves to this new stimukis, the roots growing outward in 

 the direction of the force and the stems inward, in the opposite 

 direction. The force of gravity may also be practically elimi- 

 nated, without such a substitution of another and more powerful 

 force, if the disc to which the seedlings are attached is slowly 

 revolved in a vertical plane by clockwork, thus exposing all sides 

 of the seedling successively to the stimulus. Under these con- 

 ditions, the root and the stem continue to grow in the direction 

 in which they happened to start with no reference at all to gravi- 

 tation. The pull of gravity, or any other force which we may 

 substitute for it, must therefore be able in some way to stimulate 

 the growing regions of root and stem so that growth occurs in 

 certain definite directions. It has been proven that the very tip 

 of the root, for a distance of approximately 1 or 2 millimeters in 

 length, is the only portion which is sensitive to the stimulus. If 

 this region is pointing downward, no bending of the root will take 

 place, regardless of the position of the rest of the organ. If this 

 region is horizontal, however, the root will bend downward until 

 the sensitive tip itself points downward again. The actual bend- 

 ing never takes place in the tip, however, but always in the growth 

 zone some distance behind. As to how the stimulus is perceived 

 by the tip, and how it is transmitted to the zone of bending, are 

 problems about which little is definitely known. 



Moisture. — Of vital importance to every plant is the main- 

 tenance within it of a sufficient supply of water, and we have 

 already called attention to the preponderant role which this 

 substance plays in plant functions. It comprises over ninety 

 per cent of most living plant tissues. Practically all of the 

 physiological processes of the plant are carried on in solution 

 within it. The transportation of substances from place to place 

 through the plant body is accomplished by their diffusion (in 

 solution) from one cell to another. Water is one of the two 

 essential raw materials in photosynthesis. The normal form and 

 proper functioning of the softer plant tissues is maintained by 

 keeping their cells completely filled and turgid with water. It is 

 therefore to be expected that the characteristic structures and 

 activities of plants should be concerned with obtaining and con- 

 serving an ample supply of this precious liquid. 



Definite movements and changes of position with reference to 

 moisture are shown chiefly by roots. The root tip is sensitive 

 to variations in the water-content of the soil, and will turn from a 



