44 ESSENTIALS OF BOTANY 



into mucilaginous or jelly-like portions and watery enclosed 

 droplets known as vacuoles. The liquid which constitutes 

 the vacuoles is called cell-sap. 



The cell-wall is made of a substance called cellulose, 

 which is familiar to every one in the form of cotton. A 

 bit of cotton wool is nearly, pure cellulose. Chemically it 

 is a very inactive substance, and its main use as a part of 

 the cell is to form a tough covering for it through which 

 liquids can readily soak in or out. 



58. Characteristics of Living Protoplasm.^ — The proto- 

 plasm is the active part of every cell and all the work of 

 the plant is done by the cell protoplasm, generally in the 

 higher plants, by the cooperation of many thousands of 

 protoplasts (as the little protoplasmic units are called). 

 The remarkable powers which belong to living protoplasm 

 have been summed up as follows. 



(1) The power to take up new material into its own sub- 

 stance {selective absorption). This is not merely a process 

 of soaking up liquids, as a sponge absorbs water. The 

 protoplasmic lining of a root-hair, for example, selects 

 from the soil-water some substances and rejects others. 



(2) The ability to change certain substances into others 

 of different chemical composition (assimilation or metas- 

 tasis, Sect. 166). The way in which the stored plant-food 

 of seeds is changed into the materials of the young seed- 

 ling (Sects. 9, 10) is an example of assimilative action 

 exerted by special cells in and adjoining the embryo. 

 Many other instances occur. 



(3) The power to cast off waste or used-up material (excre- 

 tion). Getting rid of surplus water and of oxygen consti- 

 tutes a very large part of the excretory work of plants. 



1 See Huxley's Essays, Vol. I, essay on The Physical ffasis of Life. 



