THE FOOD OF PLANTS. 263 



able them to give a tolerably satisfactory answer. 

 The nature of the substances employed in building 

 up the plant structure is well understood, and 

 also the form of mechanism which is adopted in 

 the first beginnings of growth, and the chemical 

 changes and transformations which occur ; but the 

 nature of the vital force which guides, and upon 

 which all activity depends, we do not understand, 

 and it is probable that human research will never 

 shed much light upon this mysterious but most 

 interesting problem. The little microscopic cell is 

 the workshop in which great changes are elab- 

 orated, and during the season of vegetable growth 

 this is the seat of the most intense activity. Every 

 plant that grows upon our earth, however great 

 or small, must be considered as having originated 

 from a single cell, so infinitesimally minute, that 

 the highest powers of the microscope are required 

 to observe it. If we turn over one of the pebbles 

 common in our brooks, we shall find a slimy ma- 

 terial of a greenish hue, adhering to its under 

 sid^. This covering is a true plant, but it is one 

 of the lowest of known forms. ' If we examine 

 it with the microscope, it will be found to be perfect 

 in structure, having an organism so wonderful as 

 to command our admiration. Feeble and insignifi- 

 cant as it is, it corresponds in structure with the 

 huge oak which grows by the stream and over- 



