40 



FUNCTIONS OF A GREEN CELL 



mineral materials may ultimately be transformed into living plant 

 protoplasm. Scientists have not determined just how this trans- 



limit, 



of 



more 



cell 

 SUvision. 



A green cell makes and assimilates food. Growth occurs. When no further growth is pos- 

 sible, the nucleus elongates, divides, and then the cell divides. Thus two cells are formed. 



formation takes place. The name given to the change of a soluble 

 protein into living protoplasm is called assimilation. True growth 

 in an individual cell is due to the increase in the sum total of the 

 amount of protoplasm resulting from assimilation. Cells may also 

 increase in size, temporarily, when the cell vacuoles become filled 

 with an abundance of watery sap. The entire plant organism 

 grows by enlargement of the individual cells, until they attain a 

 certain size. Then they divide, causing an increase in the total 

 number of cells. This process of cell division will be discussed later. 



Excretion. Any product given off through the cell wall as waste 

 is called an excretion. If a product is built up and given off for a 

 further use in the organism, it is called a secretion. Often, some 

 oxygen formed during food manufacture is held and used for oxi- 

 dation. The waste products from oxidation of sugars are carbon 

 dioxide and water. When these wastes are not retained by the 

 cell for further food making, they are excreted through the cell wall. 



Irritability. There are many substances and forces, called 

 stimuli, that act upon a green plant cell. A cell is spoken of as 

 being sensitive or irritable to light, temperature, touch, chemicals, 

 electricity, and many other stimuli. The real cause of the effects 

 produced by these stimuli is to be found in the nature of pro- 

 toplasm. It is the protoplasm that is irritable. Response is the 

 name given to the reaction the organism makes to the stimulus. 

 We have seen the effect on the protoplasm of the Elodea cell by 



