PKBFACE. 



THE idea carried out in the following treatise, namely, 

 of grouping together the phenomena of the graduated 

 organisation of plants, and those of their reproduction, 

 under a common point of view, as processes of Rejuvenes- 

 cence, and of subjecting them to a minute examination 

 under this point of view, was awakened in my mind several 

 years ago, through an investigation of Hydrodictyon, and 

 aroused anew at the end of that autumn of 1848 through 

 the discovery of the mode of reproduction of Pediastrum, 

 a genus of most elegant little Algae, which are still 

 included by many authors in the Animal Kingdom. At 

 the Annual meeting of our local Association for the ad- 

 vancement of the Natural Sciences, on the 4th of December 

 of the same year, I endeavoured, in a public lecture, 

 apropos to the description of the course of formation and 

 reproduction of the genus of Algae just named, to develope 

 this idea, and to show that it is the power of Rejuvenes- 

 cence which principally distinguishes organic from inor- 

 ganic existence, since it is to this that we must ascribe, 

 both the graduated progression of the development of the 

 individual organism, and the repetition of individuals 

 by reproduction. When in the spring of last year, the 

 confidence of my colleagues, and the grace of his Royal 



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