92 James Small on 



its own peculiar protoplasm or cytoplasm, quite independent of 

 the undoubted nuclear differences. 



In connection with the erectness of plants, our attention 

 must be directed more towards the proteins than towards the 

 other classes. Proteins have the peculiar property of acting as 

 bases or alkalies in an acidic medium and as acids in an alkaline 

 medium. It is scarcely necessary to go into further detail as to 

 how this occurs,^ but this chemical behaviour of the proteins has 

 important results. When a protein is dispersed in a fluid as 

 emulsified particles, such particles carry an electric charge, which 

 is positive in relatively acidic media and negative in relatively 

 alkaline media. The proteins themselves are slightly acidic, and 

 the medium need only be less acidic than the protein concerned 

 to be relatively alkaline ; while the medium must be more acidic 

 than the protein itself to be relatively acidic. When the 

 medium has an acidity about the same as that inherent in the 

 protein, the protein particles act neither as a base nor as an acid, 

 and they do not carry any electrical charge. This ])oint or 

 " zone " of acidity is called the isoelectric point of the protein. 



The cytoplasm of the plant cell contains such protein" 

 particles, which are lighter than the rest of the fluid. If these 

 particles are large enough, e.g. 0-0002 mm. to 0-0008 mm. in 

 radius, they will be acted upon by gravity to such an extent that 

 they will " cream " upwards fairly rapidly, just as fat globules 

 cream to the surface of milk. As calculated from Stokes' formula, 

 the rate of creaming would be about 0-002 mm. to 0-028 mm. in 

 seven minutes, which is the time taken in the case of a bean root 

 for the action of gravity to develop. its proper effect. 



How THE ItOOT GROWS DoWN. 



In the tip of every root there is a small group of cells which 

 are full of protoplasm and are undergoing active cell-division. 



i The reader will find this point and a number of related matters developed in 

 my " Text Book of Botany." Ohurohill, London. 1921. 



- Or more probably protein in loose oonibination with certain fatty substances such as 

 leoithin,. 



