PROTOPLASMIC STREAMING 23 



from the base of the organ to its apex in the course of a few Ik airs, it 

 will be realised how the plastic materials and reserve-substances 

 required for the development of a sporangium can be concentrated at 

 the proper point in a comparatively short space of time. When a cell- 

 membrane is locally undergoing growth in thickness or in surface, it is 

 quite usual for protoplasmic currents to move along those regions of 

 the wall which are engaged in growth. Criiger and Dippel have 

 observed tbis phenomenon in connection with the development of spiral 

 and reticulate thickenings. In other cases streaming may result in a 

 local accumulation of protoplasm near the region of growth ; this con- 

 dition is found in connection with such processes as the thickening of 

 the outer wall of epidermal elements, the strengthening of the ventral 

 walls of guard-cells, the protrusion and apical extension of root-hairs, 

 and so forth. 



While in all the instances so far considered the course of the 

 protoplasmic currents is controlled by internal factors, cases are 

 also known in which external influences determine their direction. 

 Thus several investigators have asserted that photic or gravitational 

 stimulation is followed by local accumulation of the protoplasm in root 

 hairs, in the statocysts of root-caps, in sporangiophores, etc. ; some of 

 these observations, however, require further confirmation. There can 

 also be no doubt that those changes in the position of chloroplasts, 

 which depend upon variations in the intensity and direction of the 

 incident illumination, are effected by definite though imperceptible 

 movements of the general cytoplasm or of special cytoplasmic fibrillae. 

 It is quite inconceivable that the chloroplasts should move about 

 actively with the aid of pseudopodium-like processes, as was suggested 

 many years ago by Schaarschmidt, and more recently again by Senn. 

 Attention may finally be drawn to TangTs discovery of the fact, that if 

 a bulb-scale of Allium Cepa is subjected to mechanical injury, the proto- 

 plasm in the cells bordering upon the wounded surface accumulates on 

 the walls which are nearest to the wound, and in so doing brings the 

 nuclei into juxtaposition with these walls. Here again it is a traumatic 

 stimulus that calls forth a movement of protoplasm in a definite 

 direction. Nestler has since shown that movements of this character 

 are of widespread occurrence. All these facts indicate that it is neces- 

 sary to distinguish between several distinct forms of protoplasmic 

 movement. The readily visible protoplasmic currents which not 

 infrequently carry chromatophores and nuclei along with them, differ 

 in nature from the imperceptible streaming that brings a nucleus into 

 an appropriate position, or that effects a transposition of chloroplasts 

 in response to photic or other stimuli. 



