The Sense Organs of Plants. 59 



one would expect all plant organs to grow downwards into the soil. 

 Instead of this, under the influence of gravity, the stem becomes 

 erect, the main root grows downwards, and the lateral roots make 

 a certain angle with the main root. Look at the matter how we 

 will, we can only conclude that the part played by gravity is merely 

 to give a kind of signal to the plant. The use of gravity to a 

 plant is analogous to that of the compass to a sailor or a plumb- 

 line to a builder. Gravity no more forces the stem of a plant to 

 become erect, than a plumb-line of itself makes a wall vertical. 

 In effect, gravity informs the plant, as the plumb-line tells the 

 builder, which direction is up, and which is down. The wonderful 

 mechanism of the plant itself does all else that is necessary. 



The question now arises, does the whole plant 'perceive' 

 gravity, or only certain parts of it ? Here again experiment comes 

 to our aid. Charles Darwin and others have shown that in the 

 case of the root, only the extreme tip has the power of perception. 

 Sir Francis Darwin has found that this is also true for the shoot of 

 grass seedlings. As Charles Darwin once wrote : ' The tip acts 

 like the brain of the lower animals.' We can thus distinguish a 

 sensory or perceiving region, the tip ; and a motor or bending 

 region some distance behind the tip. Obviously there must be 

 means of communication between these two regions, analogous in 

 some degree to the nerves of the animal body. 



The structure of the sensory root tip may now be examined, 

 to see if it throws any light on the phenomena we are discussing. 

 Many of the tiny cells of the cap which covers the growing point 

 of the root contain starch grains. These are known as falling 

 starch grains, because, unlike the starch grains of most other 

 parts of the plant, they are free to move in the cells. If, then, the 

 position of the root is altered, the starch grains also alter their 

 positions in response to gravity, and come to rest against a new 

 portion of the extremely sensitive living protoplasm. This new 

 portion we may perhaps regard as unaccustomed, so to speak, to 

 bear the weight of these minute grains. The theory is that the 

 slight pressure so set up excites or stimulates the protoplasm, 

 which thereupon transmits the stimulus to the motor region. As 



