226 Introduction. [BOOKII. 



commencement of this period (1800-1840), and important as 

 was the advance made by von Mohl towards the end of it, 

 yet we may include all that was done during that time in one 

 view, since the questions examined were essentially the same ; 

 like Mirbel and Treviranus, Moldenhawer and Meyen, von 

 Mohl was chiefly occupied up to the year 1840 in deciding 

 the questions, what is the nature of the solid framework 

 of cellulose in the plant in its matured state, whether a 

 single or double wall of membrane lies between two cell- 

 spaces, what is the true account of pits and pores, and of the 

 various forms of fibres and vessels ; one great result of these 

 efforts must be mentioned, namely, the establishment of the 

 fact that all the elementary organs of plants may be referred to 

 one fundamental form, the closed cell ; that the fibres are 

 only elongated cells, but that true vessels are formed by cells 

 which are arranged in rows, and have entered into free 

 communication with one another. 



Phytotomists before 1840, and von Mohl especially, had oc- 

 casionally paid attention among other things to circumstances 

 connected with the history of development, and single cases of 

 the formation of various cells had been described by von Mohl 

 and Mirbel between 1830 and 1840, but greater interest was 

 taken in the right understanding of the structure of mature 

 tissues ; physiological questions also, though no longer of the 

 first importance in anatomical investigations, were still of 

 weight, so far as the enquiry was influenced by the relation of 

 anatomical structure to the functions of elementary organs. 

 But with Schleiden and Na'geli the question of historical de- 

 velopment and the purely morphological examination of in- 

 terior structure assumed an exclusive prominence in phyto- 

 tomy. The first commencement of vegetable cells especially 

 and their growth were the subjects now discussed. Schleiden 

 had proposed a theory of cell-formation before 1840, which, 

 resting on too few and inexact observations, referred all 

 processes of cell-formation in the vegetable kingdom to a 



