502 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



mately translocation of the whole plasmodial mass 

 towards the source of nourishment. Some solutions 

 have an attractive or repulsive effect, according to 

 their degree af concentration. 



"5. Finally, they withdraw from light (negative 

 heliotropism). 



"With regard to the acceleration or definite direc- 

 tion of movement produced entirely by stimuli, com- 

 pare the following : 



"The knowledge of the remarkably delicate reac- 

 tion of the plasmodia under external influences enables 

 us to comprehend how these tender structures, desti- 

 tute of every kind of external protection, are able to 

 carry on their existence. The plasmodia which are 

 not yet ripe for reproduction are kept in the moist sub- 

 stratum by positive hydrotropism, which is assisted by 

 negative heliotropism. 



"But within the darkness and moisture of the sub- 

 stratum the plasmodia by no means remain in one 

 place, because the differences in the chemical compo- 

 sition of the substratum cause continual migrations. 

 The plasmodia have the faculty in a wonderful way of 

 avoiding harmful substances, and, traversing their 

 substratum in all directions, of taking up the materials 

 they require. 



"When the internal changes have proceeded so fair 

 that the plasmodia approach the fructifying condition, 

 they are brought by the negative hydrotropism which 

 now sets in, from the moist parts of the ground in the 

 forest or wood to the surface, where they creep up 

 various upright objects, often only forming rigid re- 

 productive capsules at some height from the ground. 



"When in autumn the substratum becomes grad- 

 ually colder a change which takes place from the sur- 



