PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 251 







formed from other metals. This phenomenon occurs very regu- 

 larly in the larger filaments that run along the surface of the 

 solution, and occasionally in the smaller ones. The striations in 

 the large filaments are usually about 25 microns apart (Fig. 9). 

 No observation has been made on the precise mode of formation 

 of striation in iron ferricyanide structures. Usually striation 

 indicates periodic formation; and in the case of the similar stria- 

 tions in Cd or Zn precipitation-structures a direct connection 

 with periodic precipitation can be observed. Probably the 

 conditions determining the striation in iron filaments are essen- 

 tially similar, although differing in detail. 



Mechanical obstacles placed in the path of growing filaments 

 have no appreciable influence on the regularity of striation. 

 Fine grains of sand were sprinkled into the salt solution and the 

 filaments allowed to grow out among them. The effects are 

 shown in Fig. 10. Although the filaments were broken up and 

 caused to branch, in some cases entirely enclosing some of the 

 sand particles, striation continued as before. Fig. n is a photo- 

 graph of filaments in which the direction of growth was changed 

 by placing a few drops of water from a pipette in the solution 

 just beyond the growing end of the filaments. A thin precipita- 

 tion-membrane was formed over the end of the filaments and 

 growth ceased for a moment. Soon the solution inside forced 

 its way through the thin membrane and new filaments were 

 formed which grew back into the more concentrated solution. 

 The enlarged regions of these new filaments represent osmotic 

 effects. It will be observed that striations are formed, but that 

 the distances between the striae are more variable (25 to 50 

 microns). 



2. Zinc. When a small strip of zinc, in contact with a nobler 

 metal (Cu), is placed in a 2 per cent, egg albumin solution con- 

 taining 2 per cent. KsFeCye and 0.5 per cent. NaCl the entire 

 surface becomes covered within a few minutes with a dense 

 network of filaments (Fig. 12). These structures are tubular 

 and show some cross-striation, but differ in almost every other 

 respect from the iron filaments just described. In minute 

 structure they are coarsely granular w r ith a less sharply defined 

 outline, the double contour not showing except in a few of the 



