CELLULAR STRUCTURE 31 



Protista include many other groups of simple organisms 

 of very various characters. 



As we descend the series outlined above, starting 

 with the higher (most complex) plants, we find that 

 the forms of plant life (with the exception of the fungi 

 and lichens) are more and more dependent on external 

 water, most of the algae living all their life immersed 

 in water. It is established biological doctrine that 

 the higher plants, which are able to live on dry land, 

 though their roots must in fact always be able to 

 obtain some water from the soil, have gradually arisen, 

 during the history of the world, from forms like these 

 lower plants which are confined to water. This is 

 the doctrine of organic evolution, as applied to plants, 

 a doctrine of whose truth Darwin first succeeded in 

 convincing the world. 



Cellular Structure of Plants. When we come to 

 examine their structure with the microscope, we find 

 that the bodies of nearly all plants and animals are 

 composed of what are called cells and the products 

 of cells. In the case of plants the cells are closed 

 spaces, surrounded by a cell wall of non-living substance, 

 and containing living substance called protoplasm, in 

 which all the ultimate life processes occur. This proto- 

 plasm is the only part of the bodies of living beings 

 which is actually alive. A large part of the body 

 consists of non-living organic substances formed by 

 the protoplasm. Thus in plants the body is composed 

 of a " skeleton " of cell walls, with living substance 

 in each cell cavity. The protoplasm is, however, 

 generally continuous from cell to cell by means of 

 exceedingly thin filaments of protoplasm (which cannot 

 be seen by ordinary microscopic observation) passing 



