REPRODUCTION. 243 



VI. Finally, we will briefly mention the rare occurrence of 

 * ' apogamy ' ' and related phenomena (DE BARY and his pupil FAR- 

 LOW) . It has been observed that among some ferns a plant will 

 develop from the prothallium without fertilization, hence asexually 

 (budding). According to A. BRAUN (1856), the egg-cell of Chara 

 crinita may develop into an embryo without being fertilized. The 

 egg-cell of the euphorbiaceous genus -Ccelebogyne will also develop 

 by budding. Only the female plant occurs in Europe. 



C. VARIABILITY. CONSTANCY. HEREDITY. 



The properties of a plant as a whole may be separated into 

 those which are constant and those which are variable. Con- 

 stancy and variability are clear conceptions, but their application 

 in regard to heredity, sex, and environment soon bring to light 

 great difficulties. 



Constancy is the result of heredity acting from one generation 

 to another ; the influence of both parent-plants upon the daughter- 

 plant. Variability is shown in slight differences between the 

 daughter- plants among themselves and in the differences between 

 daughter-plants and parents. (According to Nageli, heredity and 

 variability are almost inseparable ; variability depends on heredity.) 



To us variability and constancy (hence also heredity) are prop- 

 erties given to living created beings. There is no satisfactory 

 scientific definition for heredity. 



The following statements are in accordance with our present 

 knowledge of variability. There is a variability due to external 

 causes, such as the influence of the surrounding medium, climatic 

 conditions, etc. ; and a variation due to internal causes which can- 

 not be perceived externally. The latter causes are least under- 

 stood. The external causes can only be interpreted teleologically ; 

 we are unable to give them an ultimate causal explanation. We 

 will cite an example of variation due to external causes. Poly go - 

 num amphibium usually grows near the margins of ponds, but may 

 also occur on dry land. It has been observed that the anatomical 

 structure of the land-form is different from that growing in water. 

 In the former the intercellular aerating system is slightly developed, 

 while the vascular system is strongly developed. In the water- 

 form the reverse is true ; the air-chambers are large, which insures 



