HAPTOTROPISM 491 



first of all, by employing a 14 per cent, gelatine solution which was kept moist, 

 to fix bodies to the tendrils under investigation without inducing stimulation ; 

 further the gelatine was smeared over a glass rod and this rod was employed in 

 the study of the influence of different types of contact on tendrils. By its means 

 the effect of constant pressure, both uniform and gradually increasing, was 

 tried ; solitary or numerous successive blows, light or heavy, were inflicted on 

 the tendril, or the tendril was rubbed with the rod ; in no case, however, did 

 any stimulation occur, nor did any curvature follow after constant pressure 

 or blows, single or successive, or after friction ; each and all were quite ineffective. 



In the course of his experiments PFEFFER proved that blows, administered 

 by a solid body (apart from gelatine) were stimulants provided they were of 

 sufficient intensity. Thus thin smooth glass threads, sticks of wax, filter paper, 

 animal membrane in the dry and the wet condition induced a positive reaction, 

 and the significance of the velocity of the impacts could be easily demonstrated 

 by means of particles of clay suspended in water. On the other hand it was seen 

 that solid bodies induced no reaction if the pressure they exerted were statical, 

 that glass threads or needles, if they were pressed against the tendrils cautiously 

 and without any friction and without any sudden increase in the pressure, 

 produced no effect. Nor was there any result when a short piece about 4 mm. 

 long of the tendril was subjected to a constant pressure of a solid body, on 

 which there were several different points of contact (e.g. a rusty nail, emery 

 paper). In all these cases, however, a reaction took place at once when the 

 bodies in question were gently rubbed on the tendril no matter how limited the 

 surface of contact was. Of the greatest importance is the fact that small blows 

 of this kind, though unable to induce a reaction when administered singly, 

 induced curvature by summation of stimuli. The tendril remains sensitive for 

 a remarkably long time to constantly recurrent stimuli, and while the reaction is 

 in full progress new stimuli may be perceived until gradually the tendril 

 becomes accustomed to them. 



As a result of his inquiries PFEFFER summarized the perceptive powers 

 of tendrils in the following words : ' In order that a stimulus may be effective 

 definite points of limited extent in the sensitive region of the tendril must be 

 affected by a push or a pull of sufficient intensity, simultaneously or in adequately 

 rapid succession. On the other hand, the tendril does not react as soon as the 

 blow affects all points of a larger surface with approximately equal intensity 

 in such a way that compression of closely adjacent regions is sufficiently nearly 

 uniform* (gelatine). Thus it is that tendrils are not excited either by mechanical 

 shaking in general or by rain. ' Under all conditions a local compression, 

 decreasing with sufficient rapidity, is a condition of stimulation, which is 

 not induced by statical pressure only, even if such a pressure affects widely 

 separated parts with considerable intensity.' 



In order to have a single word to express the mechanical conditions of 

 tendril stimulation we will use PFEFFER' s term, contact, although that word 

 might be more appropriately applied to statical pressure ; we may say, in 

 a word, that tendrils possess contact sensitivity. We shall find that other 

 bodies as well as tendrils possess the capacity of responding to contact-stimulus. 

 Whether or not a similar mechanical influence leads to perception in the interior 

 of the cells in the case of geotropism, it is impossible at present to say ; there, 

 apparently, statical pressure does act as a stimulus. 



We must assume that the perception of the stimulus takes place in the 

 epidermis, and PFEFFER has shown in this relation that the deformations of the 

 protoplasm necessary for perception might be simplified in many tendrils by 

 certain histological arrangements such as the so-called 'sensitive pits '. Still 

 it must be remembered that there are many sensitive tendrils which do not 

 possess such histological differentiations, so that these pits cannot obviously 

 be a necessary condition of stimulation. It is needless therefore for us to 



