LECTURE XIII. 



THE MOLECULAR STRUCTURE OF PLANTS, AND ITS 

 PHYSIOLOGICAL IMPORTANCE. 



The coarser structure of plants as perceived with the unaided eye in their 

 external form, as well as the structure visible with the microscope, have been 

 treated of in previous lectures ; and, as occasion offered, reference has also been 

 made to the fact that the explanation of the phenomena of life (that is physiological 

 investigation) cannot be satisfied with the knowledge of this visible structure, 

 but that we are obliged to form more definite conceptions of that structure which 

 is no longer visible, and no longer perceivable even with the strongest powers 

 of the microscope. The attempt may now be made, preparatory to what is to be 

 said later, to bring forward here some of the most important and most general 

 results of investigation in the latter direction. The problem is to draw conclusions 

 from the phenomena evident to the senses, which give us definite information as to 

 the structural relations no longer perceivable by the senses. In such cases it is 

 always advisable not to build conclusions on conclusions, and hypotheses on 

 hypotheses ; but to draw, from safely established facts only, the conclusions which 

 immediately follow. At the same time it will be well, for the better guidance of 

 those who are not quite at home in scientific matters, to go back somewhat further 

 than may appear necessary. 



Not only physiological, but physical and chemical investigation also, were 

 long ago driven to form certain conceptions with respect to the minute invisible 

 structure of bodies, in order to obtain a more definite insight into natural processes. 

 In chemistry this is done by the assumption of the existence of atoms — exceedingly 

 small, indivisible masses of matter with which the chemical forces are associated. 

 It is imagined that the chemical properties of an elementary substance such as 

 Hydrogen, Oxygen, Potassium, Phosphorus, etc., are still existent even in atoms 

 of these substances. Certain chemical phenomena, however, necessitate the as- 

 sumption that two or more atoms of an element may come together into a closer 

 combination, which is distinguished as a molecule. Chemical combinations of 

 different elements must always necessarily be composed of two or more atoms, 

 and therefore always form molecules. A molecule is, therefore, according to the 

 view of the chemist, the smallest conceivable mass of a chemical compound ; 

 since, if the molecule were split up still further, the chemical nature of the object 



