6 BOTANY: PRINCIPLES AND PROBLEMS 



plant bod}^, and their works on "phytotomy " laid the foundation 

 for our modern knowledge of plant morphology. The continued 

 improvement in the compound microscope made possible more 

 complete and accurate knowledge of the way plants are con- 

 structed, and led to the formulation bj^ Schleiden, in 1838, of the 

 Cell Theory, which states that the cell is the unit of structure in 

 plants and that protoplasm is its essential constituent. From 

 this beginning, modern anatomy and cytology have added a great 



Fig. 2.— Carolus Linnaeus (Carl von Linne), 1707-1778. 



body of facts to our knowledge of the structure, growth, and 

 reproduction of the plant body. 



The ancients and the early modern botanists for the most 

 part had fanciful and inaccurate ideas as to the way in which the 

 plant carried on its various functions, an ignorance largely due 

 to the undeveloped state of the sciences of physics and chemistry 

 at the time. It was not until the latter part of the eighteenth 

 century that modern plant physiology became definitely estab- 

 Hshed. Oxygen was discovered by Priestley in 1774, and five 

 years later a Dutch physician, Ingenhousz, observed that green 

 plants in the light take in carbon dioxide and give off oxygen, 

 and that all plants give off a certain amount of carbon dioxide. 

 These gas exchanges were accurately measured by de Saussure 

 in the early years of the nineteenth century, and some of the 



