IRRITABILITY 203 



than elsewhere, a readily oxidized substance, perhaps also 

 another which is a vehicle of oxygen. In the irritated root- 

 tip, and directly in proportion to the irritation, the amount 

 of aromatic oxidizable substance increases and the substance 

 or substances serving as a vehicle for oxygen decrease. This 

 transfer of oxygen from one compound to another within 

 the cell is accomplished not by gravity but by the living 

 protoplasm stimulated by gravity. 



The transmission of a stimulus which caused only mechan- 

 ical changes in the stimulated cell would be very difficult to 

 conceive. On the contrary, when there is a change in the 

 amount or the composition of diffusible substances, it is 

 much easier to form some notion of the means by which the 

 impulse to grow in a definite way is given by the meriste- 

 matic cells at the root-tip to those in the growing region 

 behind. Any change of this sort in one cell is necessarily 

 followed hj corresponding diffusion currents between this 

 cell and its neighbors more and more remote. Wherever a 

 new substance enters the living protoplasm of a cell it will 

 affect the protoplasm chemically or physically, thus pro- 

 ducing a stimulus. The irritated periblem cells of the 

 root-tip give up by diffusion the changed substances 

 which they have formed. Finally those cells in the grow- 

 ing zone are reached by the diffusing compounds, and 

 they change the direction if not also the rate of growth 

 of the organ. 



Diffusion undoubtedly occurs between the sensitive cells of 

 the root-tip and the cells more or less remote. The trans- 

 mission of an impulse by diffusion alone is not rapid, and 

 it is difficult to prove. To avoid both of theser objections 

 Nemec* has recourse to protoplasmic fibrils to which he 

 attributes the function of conducting stimuli from cell to 

 cell. These fibrils can be seen in suitably prepared fixed 

 material and in certain living cells (e.g. stamen and sHme 

 hairs of Tradescantia) , both in tissues which are irritable 

 and which exhibit visible reactions to stimuli, and also in 

 tissues in which no response to stimuli has ever been de- 



* Nemec, B. Die Eeizleitung und die reizleitenden Structuren bei den 

 Pflanzen. Jena, 1901. 



