CHAP, in.] of Cell-membrane in Plants. 273 



' Exposition et defense de ma thdorie de 1'organisation vegetale,' 

 in which Mirbel endeavours to meet the objections of his 

 opponents with great adroitness of style and with the results 

 of varied rather than profound observation, and to find new 

 arguments for his theory of vegetable tissue ; he admits that 

 his former treatises were in many respects faulty, but demands 

 that his critics should discuss his system as a whole and not 

 take offence at single expressions. Mirbel's idea of the inner 

 structure of plants is essentially the same as that broached by 

 Caspar Friedrich Wolff. The first and fundamental idea is 

 that all vegetable organisation is formed from one and the 

 same tissue differently modified. The cell-cavities are only 

 hollow spaces of varying form and extension in homogeneous 

 original matter, and have no need therefore of a system of 

 filaments, as Grew supposed, to hold them together. The 

 tracheae only are an exception, which Mirbel, in striking 

 opposition to the much more correct view of Treviranus, 

 considers to be narrow spirally wound laminae, inserted into 

 the tissue and connected with it only at the two ends. If it is 

 asked how interchange of sap is effected in such a cellular 

 tissue as this, it becomes necessary to assume that the mem- 

 branous substance of plants is pierced by countless invisible 

 pores, through which fluids find their way. But nature has a 

 speedier and more powerful instrument in the larger pores, 

 which the microscope reveals. Mirbel does not discuss the 

 question how the fluids are set in motion, easily disregarding 

 such mechanical difficulties, as was usual in those days, when 

 vital power was always in reserve to be the moving agent. 

 He warmly repels the imputation, which Sprengel had made 

 against him, of having confounded pores and granules, by 

 appealing to his figures ; he says that he has depicted pro- 

 minences on the outside of the walls of the dotted vessels, and 

 an orifice in each of them, which his opponents simply never 

 saw. The question whether these prominences lie on the 

 inside or the outside of the walls of the vessel has no meaning, 



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