EXCITATION AND INHIBITION 



431 



It is found that a stimulus, such as gravity or light, requires to act for not less 

 than a certain time in order to have any effect at all. This minimum time is 

 known as the " presentation time." Further, however long a stimulus is applied, 

 no effect is produced until an interval of time has elapsed since the beginning of 

 the stimulation. This is the "reaction time." 



There are no special channels for conduction of excitatory processes like the 

 nerves of animal organisms. Conduction appears to take place through the cell 

 protoplasm, which must be living. In the stem of Tradescantia virgitnca, the 

 curvature which takes place under the stimulus of gravity takes effect on the 

 next internode below the one stimulated, and the transmission is abolished by 

 local anaesthesia of a spot between the two. Similarly, light stimulus acting on 

 the stem of the Dahlia is transmitted to the root. A curious fact, whose probable 

 explanation will be given in Chapter XVII. on receptor organs, is 

 that, at the temperature of or in an atmosphere of hydrogen 

 or carbon dioxide, no effect is produced until the temperature 

 is raised or oxygen supplied, respectively. But, although the 

 actual stimulus may have ceased before the change of conditions, 

 the effect shows itself. 



When light stimulus and gravity stimulus act together, the 

 former, as a rule, completely overpowers the latter. 



The manner of conduction has been a subject of dispute. In 

 the case of the sensitive plant, it was originally held by Pfeffer and 

 by Haberlandt that the transmission was mechanical, by a move- 

 ment of water in tubes of the vascular bundles ; but the abolition 

 of power of conduction by local anaesthesia is strong evidence that 

 it takes place through protoplasmic structures. It is known in 

 many cases that the protoplasm of neighbouring plant cells is united 

 by strands passing through holes in the cell walls (see especially the 

 work of Gardiner, 1884). Fig. 131 shows the structure of a tissue 

 of this kind. Further details as to the mechanism of conduction 

 will be found in the essay by Fitting (1906). 



In connection with the increase of per- 

 meability which we have seen to occur in 

 the state of excitation, the mechanism of the 

 movements of the sensitive plant, investi 

 gated by Pfeffer (1873), is of interest. As 

 we have seen, the vegetable cell is main- 

 tained in a state of turgor by means of the 

 osmotic pressure due to the presence within 

 it of substances in solution, and to the im- 

 permeability of the cell membrane to these 



solutes. Since the cell wall surrounding each is incapable of any considerable 

 stretching, a pressure in the interior results. A mass of cells with such a turgor 

 well developed exists at the lower side of each movable joint in the leaf of the 

 plant. The cells on the upper side are less turgid. When stimulated, the cell 

 membrane of the lower cells suddenly loses its semipermeable character, as 

 regards the solutes of the cell contents, with the consequence that the internal 

 pressure can release itself by filtering solution through the membrane. Drops 

 appear on a cut surface and the weight of the leaf, being no longer supported 

 by the distended cells, causes it to fall. 



Another phenomenon, which may be in some way connected with changes of 

 permeability, is the oxidation reaction, described by Czapek and Bertel (1906). If 

 longitudinal sections are cut from the root point of lupin seedlings, it is found that 

 their cells become brown on boiling with ammoniacal silver nitrate, owing to the 

 presence of a reducing substance. If the root has been stimulated geotropically, 

 the dark stain is more intense. Investigation showed that this reducing reaction 

 was due to the presence of tyrosine, although indirectly, being actually given by 

 homogentisic acid, apparently produced by the action of an enzyme on tyrosine. 

 Now the difficulty is that tyrosine has an OH group in the para-position as regards 



FIG. 131. SIEVE TUBE. 



The protoplasts, contracted by the action 

 of alcohol, adhere to the transverse 

 wall, and that of each cell is con- 

 nected to the other by delicate proto- 

 plasmic filaments, passing through 

 the pores of the cell wall. 



Magnified. 



(From Timiriazeff. ) 



