5^ Professor R. H. Yapp on 



periodicity is seen in the 'sleep movements' of the leaves of the 

 wood-sorrel and clover, while the rapid closing of the leaves of 

 the sensitive plant (Mimosa) when touched, is even more striking. 

 These examples show that plant organs have the power of 

 movement. But the movements I want to consider to-night are 

 the slow growth movements which result in the placing of the 

 various plant organs in the positions where they can best do their 

 special work. These movements, like those first mentioned, are 

 carried out in response to various stimuli, or influences brought to 

 bear on the plant from the outside. 



Let me remind you that the chief duties of a root are to 

 anchor the plant, and to absorb water and mineral salts : it should 

 therefore be placed in the soil. Leaves, on the other hand, 

 manufacture food, and to do this they need light and air. They 

 must be in the air above the soil. Bearing these facts in mind, 

 we may start with a germinating seed. Everyone knows, though 

 the fact is none the less remarkable on that account, that the 

 shoot of a seedling invariably grows upwards into the air, while 

 the root grows downwards into the soil. Whichever way up the 

 seed is planted, the result is the same. How can the seed tell, 

 especially when deeply buried in the earth, which is the up, and 

 which the down direction ? Older plants, too, have the same 

 power. If a plant with an erect stem is placed horizontally (in a 

 dark room, so as to exclude the possible influence of light), the 

 stem will bend as it grows, and once more assume the vertical 

 position. Similarly, the main root will curve, and again grow 

 vertically downwards. This adjustment is a gradual one, the 

 stem or root in its growth often swinging past the vertical, then 

 backwards and forwards like a pendulum till the required position 

 is attained. 



Experiments have shown conclusively that this ability to 

 perceive direction is in some way connected with gravitation. 

 Gravity can actually influence the direction of growth of plant 

 organs. We must be clear as to what is implied in this statement. 

 It is evident that gravity does not act mechanically on the mass 

 of the plant, as it would on a piece of lifeless matter : if it did 



