260 SUMMARY OF CURRENT RESEARCHES RELATING TO 



is to determine the nature and the causes of the sensitiveness to con- 

 tact, the set of movements termed by Errera haptotrojrism. 



The author found the sporangiophore of Phycomyces excessively 

 sensitive to slight continuous contact, the hcliotropic movements being 

 comparatively very sluggish. A contact of from three to six minutes is 

 sufficient in most cases to incite a decided curvature, concave on the side 

 of contact ; when the pressure ceases, the curvature may either remain 

 or continue. This sensitiveness is limited to the growing region of the 

 cell ; in mature plants no curvature takes place. The strongest curva- 

 ture is not, however, necessarily at the point of contact, but always at 

 the point of most vigorous growth, but it must always commence at the 

 exact point of contact. Phycomyces behaves, in fact, exactly like a 

 growing tendril. 



As in geotropic, hcliotropic, and hydrotropic curvatures, so these 

 haptotropic curvatures of Phycomyces are always accompanied by a 

 peculiar change in the distribution of the protoplasm, which accumu- 

 lates on the concave as contrasted with the convex side. This change 

 commences as soon as the curvature begins, and when this ceases the 

 protoplasm again assumes its uniform distribution. This accumulation 

 of protoplasm on the concave side of the cell is unquestionably the 

 result of an actual transfer from one side to the other ; and it is always 

 accompanied by an increase in the thickness of the cell-wall. The cell- 

 wall may be more than twice as thick on the concave as on the convex 

 side. This increase in thickness of the cell-wall is the cause of the 

 curvature, increasing its elasticity and decreasing its extensibility. 

 The cell-protoplasni behaves, in fact, like a free plasmodium until its 

 motion is arrested by the cell-wall where it accumulates. This is 

 followed by a change in the constitution of the cell-wall, and this again 

 by the action of tnrgidity on the altered cell-wall. 



Similar results, in their main features, were obtained with other 

 unicellular organs, such as the cells of Saprolegnia, root-hairs, &c. 



Passing now to the consideration of the corresponding phenomena in 

 multicellular organs — roots, stems, nodes of grasses, leaf-stalks, tendrils, 

 climbing stems, &c. — no such accumulation of protoplasm can be 

 detected on the irritated side of the separate cells ; the alternative that 

 the entire multicellular organ acts like a single cell can be proved to be 

 the correct one. On the concave side of the organs named, the cells are 

 always found to contain more protoplasm than those on the convex side ; 

 and the same is true of roots which display geotropic curvatures ; and 

 the accumulation of protojdasm is in proportion to the degree of 

 curvature. 



This change in the distribution of the protoplasm in a multicellular 

 organism can only take place by tlie transference of the protoplasm 

 from cell to cell, and hence necessitates the assumption of a continuity 

 of protoplasm throughout the organ, an assumption the correctness of 

 which the author has amply demonstrated, both in the cortical and in 

 the medullary parenchyma ; this continuity is especially clearly seen in 

 the growing points immediately behind the apex of the root. The 

 irritation of the organ in question acts therefore not on a number of 

 protoplasts, but on a single one capable of passing through the perfora- 

 tions in the cell-walls. Just as in unicellular organs, this accumulation 

 of protoplasm is accompanied by an increase in the thickness of the 

 cell-wall ; and this thickening is so energetic that it is perfectly visible 



