IRRITABILITY TO CONTACT AND TO MECHANICAL SHOCKS 67 



The shape and relationships of the cell and cell-wall, as well as the way in which 

 the cells are joined and arranged in the tissues, may render the perception of the 

 stimulus more readily possible at particular points, but do not produce this special 

 form of irritability. The production and activity of the response are, however, 

 dependent in a much greater degree upon the structure of the organ, but the primary 

 perception always takes place in the sensitive protoplasm. The impermeability of 

 the cell- wall or of an intervening tissue may render it difficult or impossible for a sub- 

 stance to exert any chemical stimulation, or may restrict its action largely or entirely 

 to those points where the substance is able to penetrate. Differences in the trans- 

 parency of the tissues must act in the same way in regard to light stimuli, and hence it 

 arises that a seedling performs a heliotropic curvature in diffuse light if one side is 

 covered with indian ink. Similarly the presence of thick walls, or of resistant tissues, 

 may render the sensitive cells beneath less responsive or not responsive at all to blows 

 or pressure. Furthermore, the arrangement of the tissues may be such that pressure 

 and tension exercise different stimulatory actions, or may cause contact at a particular 

 region to produce a response especially readily as in the case of the sensitive haiis of 

 Dionaea. This is probably because pressure at these points is more readily trans- 

 mitted to the sensitive cells beneath \ The best knowledge of the structure of an 

 irritable organ will not reveal the nature of irritability, and in fact organs with a pro- 

 nounced similarity of structure may possess widely dissimilar irritabilities, while the 

 same sensitivity may be shown by organs differing widely in structure. Furthermore, 

 various special irritabilities may reside in cells and tissues which differ in no anatomical 

 features from ordinary indifferent cells and tissues 2 . It <is also easy to see that the 

 coarser anatomical structure can more readily favour the perception of mechanical 

 stimuli, than of thermal or photic stimuli ; and the observed facts bear out this con- 

 clusion. It must, however, be remembered that the mere enumeration of all the 

 observed cases in which the anatomical structure shows a biological adaptation for 

 the reception of stimuli leads one to attach undue importance to structure, and as 

 a matter of fact in most cases the structure shows no perceptible adaptation for 

 sensory perception. In any case physiology is only concerned with structure in so 

 far as it affects functional -activity 3 . 



SECTION 15 (continued). 



Since the distinction between seismonic and contact irritability is 

 purely a matter of special sensitivity, it remains an open question whether 

 both forms of stimuli involve similar or dissimilar primary reactions. 

 Seismonic stimulation usually produces variation movements, but contact- 



1 The first interactions may be purely physical or chemical, and may act as a preparation for the 

 subsequent physiological perception. When purely mechanical transmission is performed by hairs 

 or the like, Haberlandt (Sinnesorgane im Pflanzemeich, 1901, p. 9) terms the intermediary structures 

 4 stimulators.' 



t a All cells and organs capable of perceiving stimuli may be termed sense-organs, whether they 

 show any special anatomical structure or not. 

 8 On problems of this kind see Haberlandt, 1. c., 1901. 



