TISSUES AND SIMPLE ORGANS. 49 



the root and stem. These three formative tissues or histogens are 

 the dermatogen (for the epidermis), periblem (for the parenchyma), 

 and plerome (for the central tissue, vascular system). Though this 

 question is still undecided, it is certain that the sharp distinction 

 between " dermatogen," " periblem," and " plerome " cannot be 

 demonstrated in many cases. According to SCHWENDENER the 

 existence of the plerome as a special histogen has not been definitely 

 proved in a single instance. 1 



B. DIFFERENTIATION OF TISSUES ACCORDING TO STRUCTURE AND 



FUNCTION. 



(PHYSIOLOGICAL ANATOMY OF SIMPLE ORGANS.) 

 Differences of Functions and their Enumeration. 



Although the physiological method of investigation as applied 

 to the anatomy of plants (hence called the anatomical-physiological 

 tendency) was not unknown in the year 1874, our knowledge con- 

 cerning the relations of the anatomical structure to the life of the 

 plant was so imperfect that anatomy in general was almost entirely 

 separated from life-processes and became a matter of mere " dead " 

 description. The question : Why ? in other words : Why are va- 

 rious tissues and cell-forms so arranged and formed ? was made 

 applicable to the most widely different internal organs of plants 

 after SCHWENDENER in his work " The Mechanical Principles of 

 Stem-structure," etc., had given an important tissue-system a care- 

 ful consideration from the teleological standpoint. By it the teleo- 

 logical method of investigation received a strong impetus. These 

 preliminary remarks are in order, since in them lies the justifica- 

 tion of the arrangement of the greater part of this book, especially 

 for the extensive discussion of tissues. 



We shall now consider the important tissues according to struc- 

 ture &nd function. 



General considerations will show what functions predominate in 

 plant-life and why plant-organs must exist. 



The life of the plant manifests itself in three ways. These 

 life-manifestations are : nutrition, growth, and reproduction. A 

 series of special functions are dependent upon these three great life- 

 activities. 



1 Sitzungsberichte der Berliner Akad., 1882. 



