THE DISPLAY OF ENERGY 371 



In spite of the work of Sir J. C. Bose, the distinguished Indian 

 botanist, who used very dehcate instruments to measure tlie irrita- 

 biUty of plants, scientists as a group have not accepted his behef that 

 t\\o transmission of stimuh in plants is by means of a mechanism 

 similar to the nerves of animals. There is no doubt that certain 

 parts of the plant stem do conduct stimuli more rapidly than others, 

 but it is doubtful whether the conducting strands of protoplasm in the 

 sieve tubes of the phloem are actually the areas of special transmission. 

 Experiments have been made in which the stimulus of an electric 

 current can be cut out by the use of anesthesia, just as in the case of 

 the nerves of animals, but since the cells in the area where the stimulus 

 is transmitted are much shorter than the neurons in the animal, 

 transmission is naturally slower and anesthetics have the same 

 effect on living protoplasm in each case. One investigator, Ricca, 

 has shown that a stimulated region of a plant secretes a hormone that 

 travels to the region of response, causing a reaction to the stimulus. 

 Other workers have even shown that if the tip of one plant is grafted 

 to another plant from which the tip has been removed, the stimulus 

 will be transmitted to the responsive region of the latter plant. A 

 number of experiments upon plants indicate that stimuli are trans- 

 mitted by means of hormones which are carried in the transpiration 

 stream through the vascular bundles. Too little is knawn at the 

 present time to say with certainty exactly what effect hormones 

 have, but it is quite evident that they do play a part in the trans- 

 mission of stimuli. 



One of the most studied responses is geotropism. Roots are 

 assumed to respond positively to the pull of gravity while stems are 

 considered to be negatively geotropic. Branches and leaves usually 

 grow at right angles to the force of gravity while some roots place 

 themselves at a definite angle to this force. Gravity has been shown 

 to be a stimulus by experiments which either replaced it by some other 

 force, or neutralized its effect. For example, plants are placed on a 

 slowly revolving disk called a clinostat. If the })lant is revolved 

 horizontally on the disk, which rotates parallel to the long axis of the 

 plant, the roots and stems will continue to grow in the same direction 

 as they did at the beginning of the experiment. Gravity in this case 

 acts on all sides of the plant eciually, with the result that there is no 

 change in the position of the plant's organs. In the famous experi- 

 ment of Thomas Andrew Knight, who worked in the early part of the 

 nineteenth century, plants were placed on a rapidly rotating disk in 



