THE NATURE AND REACTIONS OF PROTOPLASM. 11 



With regard to the influence of heat, it has been determined 

 that a certain temperature, which varies for different cells, is 

 required, in order that the protoplasm maj' carry on to the best 

 of its ability the complex processes involved in the manufacture 

 of nitrogenous and carbohydrate food; and it is a well-established 

 fact that chloroplasts in the cells of green parts of plants are 

 markedly affected by light (see Chap, x.), these chloroplasts being 

 in the main protoplasmic in nature (ie., specialised portions of 

 the protoplasm). 



Other physical agencies, such as gravity and moisture, have 

 a powerful "directive action" upon the protoplasm of cells of 

 the growing-point of roots and stems; while certain chemical 

 substances (enzymes and malic acid) have a marked influence in 

 causing the attraction of swarmspores and the growth of pollen- 

 tubes. This attraction is known as "positive chemotaxis." 

 With regard to the movement of the protoplasm round a cell 

 (so-called "streaming" or "rotation," see Chap, ii.), Hofmeister 

 regarded this as depending upon variations in the absorptive 

 capacity for water shown by the living substance at different 

 points of a cell ; in this case also, it is necessary to take into 

 account the influence of temperaturjp, and possibly differences in 

 electrical potential at various points in a cell. The phenomeiui 

 of surface tension may, however, account for some of these 

 protoplasmic mo\'ements. 



Experimenting upon the vitalitj' of seeds, one investigator * 

 discovered that those capable of germination were, when stimu- 

 lated by an electric current, also capable of producing a so-called 

 " hlaze-reaction '' — viz., an electric response-current in a definite 

 direction when included in circuit with a sensitive galvanometer 

 — and he showed that this current was evidence of the vitality 

 of the protoplasm of the seeds experimented upon. The reaction 

 was in all probability due to chemical changes set up by stimula- 

 tion in the living substance, of the nature of slow oxidations, 

 giving rise to changes in electrical potential. In the above 

 experiments it was found, moreover, that if the " water of consti- 

 tution " in the protoplasm were first of all removed by drying at 

 high temperatures, or alcohol, no blaze-reaction resulted, pointing 

 definitely to the fact that this loosely combined water was essen- 

 tial to the maintenance of vitality. Another factor which is also 

 * Prof. Waller. 



