8 A TEXTBOOK OF THEORETICAL BOTANY 



for independence of each cell depend on the extent of the functions which 

 devolve upon it in the process of subdivision, but so long as it remains a 

 part of the structure of the higher organism, it remains also functionally 

 integrated into the life of the whole. 



In the simplest multicellular organisms all the cells are seemingly 

 equivalent and all are potentially independent, but they are nevertheless 

 integrated parts of a whole, and this is seen from the fact that even in these 

 simple cases potential differences may exist which are only revealed at 

 certain times or under certain circumstances. Every advance from this 

 level has been associated with increasing differentiation between cells, at 

 first purely functional, but later structural as well, leading at length to such 

 high degrees of specialization that even the bodies of dead cells may be of 

 essential service to the organism. Differentiation is not something added 

 to organization, it is a necessity of organization, and differentiation of 

 cells is inherent in the nature of the multicellular condition even at its 

 simplest. 



The essential material in every living cell is the protoplasm, the wall 

 being no more than a protective covering secreted by it, but protoplasm 

 was not recognized as the living substance until 1835, when it was named 

 " sarcode " by Dujardin. Its universal importance was announced by 

 Ferdinand Cohn in 1850, and the name protoplasm, used a few years before 

 by von Mohl, gradually replaced the earlier term. Curiously enough the 

 discovery of the cell nucleus by Robert Brown in 1831 came before that of 

 protoplasm. Although the nucleus is itself protoplasmic, it is usually denser 

 and is more easily seen than other parts of the living matter. 



That cells are individual entities, the bricks out of which all living bodies 

 are built, was the view advanced by Schleiden and Schwann, about the 

 year 1838, under the title of the Cell Theory. Its truth has long been firmly 

 established, and every investigation has only strengthened our belief in the 

 fundamental character of the cell as the unit of living organization in all 

 higher plants and animals. Whether unicellular organisms are equivalent 

 to single cells of a tissue, or to complete organisms, is a moot point. All we 

 can say definitely is that the organization of protoplasm into self-centred 

 units is co-extensive with life itself. In fact we know nothing of life as such, 

 we only know living organisms. 



The essential structures comprised in a plant cell (Fig. i) may be 

 summarized as follows : — 



1. The Cell Wall, basically composed of cellulose but often chemically 

 altered by the incorporation of other materials. This encloses a space, the 

 Cell Lumen, in which the following occur. 



2. The Cytoplasm, which includes all the protoplasm outside the nucleus, 

 with certain differentiated structures : — 



The Plasma Membrane, an external surface layer in contact with the 

 wall. 



The Vacuolar Membrane or Tonoplast, bounding the vacuoles. 



