CHAP, in.] of Cell-membrane in Plants. 275 



the word cambium not for the layer of tissue afterwards so 

 called, but for a highly ' elaborated and purified sap ' which is 

 intended for the food of the plant and makes its way through 

 all membranes ; we see this cambium-sap appear at the spots 

 where it produces new tubes and cells after the manner of the 

 Wolffian theory. The cells appear at first as minute spheres, 

 the tubes are very fine lines ; both enlarge and gradually show 

 pores, clefts, etc. This is essentially Wolff's doctrine, which 

 Mirbel afterwards endeavoured to confirm against his German 

 opponents from the germination of the date-palm with the 

 help of a more powerful microscope. 



Mirbel insisted more than the German phytotomists of his 

 day on the idea, that all forms of vegetable tissue are developed 

 originally from young cell-tissue, an idea suggested by Sprengel 

 and following naturally with Mirbel from Wolff's theory. Both 

 Mirbel and Wolff were hasty in observation and too much 

 under the influence of theory in giving reasons for what they 

 observed, and therefore too ready with far-reaching explanations 

 of phenomena which only long-continued observation could 

 decide. 



Treviranus replied, though after some delay, to Mirbel's 

 polemics by incorporating into his ' Beitrage zur Pflanzen- 

 physiologic,' G6ttingen(i8n), an essay entitled 'Beobachtungen 

 im Betreff einiger streitigen Puncte der Pflanzenphysiologie,' 

 in which he again took up the questions in dispute between 

 himself, Mirbel, Link and others, and supported his own views 

 by fresh investigations. It cannot be denied that in this short 

 treatise Treviranus brought some important questions nearer 

 to a decision; he added materially to the knowledge of 

 bordered pits, on which subject his views now approximated 

 more nearly to those of Mirbel ; he drew attention to the 

 vesicular nature of vegetable cells, which are often separable 

 from one another, and to the occurrence of true spiral vessels 

 in the neighbourhood of the pith in Conifers also, and among 

 other things discovered the stomata on the capsule of Mosses. 



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