INTRODUCTION XXV 



The causes that produce certain phenomena and the conditions that first 

 render them possible must also be differentiated. For instance, if solid calcium 

 sulphate is mixed with solid barium chloride there is no reaction; when water 

 is added, however, barium sulphate and calcium chloride are formed. This 

 reaction is caused by the chemical attraction of the elements, the water acting 

 only as a necessary condition. Thus releases, which are conditioning factors, 

 must be distinguished from real causes." 



Plants have an internal structure, being composed of cells of various forms 

 and sizes. The life of an organism is the sum-total of the life activities of the 

 individual cells composing it, and the study of plant physiology presupposes an 

 acquaintance with the internal structure or anatomy of the plant. Familiarity 

 with the microscope is essential in physiological study, since many important 

 physiological questions can be solved by its use. 



For the study of many physiological phenomena — those of growth and en- 

 largement, for example — a knowledge of the structure of the given plant and an 

 acquaintance with the external conditions affecting it, are not sufficient; it must 

 also be remembered that the plant has developed from a long series of ancestors 

 whose form and mode of living has not been without effect upon the offspring. 

 In these cases, therefore, heredity must be taken into account.-'' 



• The definition of the term cause involves difficulties. It is probably best to consider that 

 all changes are determined (in quantity, rate and direction) by a set of controlling conditions, 

 the cause — in the ordinary sense — being simply the last one of these necessary conditions to be 

 fulfilled. For a discussion of this matter see : Verwom, Max, Kausale und Konditionale Welt- 

 anschanung, Jena, 1912. — Ed. 



f This is somewhat vague; the phenomena in question are assuredly conditioned at any 

 given time by the internal and external conditions then prevailing. The nature of the ances- 

 tors of a plant and the surroundings under which these lived are but secondary conditions, 

 which have been influential in determining what are the present internal conditions (what the 

 plant is now), but which are, in themselves, without any present direct influence upon its 

 processes. The phenomena connoted by the term heredity have played an important rfile in 

 determining the present internal conditions, and these latter, together with the present sur- 

 roundings, are now influential in the determination of physiological phenomena. — Ed. 



