16 



tensity, at least the velocity of the motion in the cells does 

 not depend on the degree of heat. 



M. Morren* has published sx>me views on the circulation 

 of the sap in Dicotyledons, in which he observes that Mohl's 

 discovery of an intercellular substance must lead to a more 

 correct idea respecting the organs of circulation. But it has 

 many years ago, in my opinion, been proved by several phy- 

 siologists that the intercellular passages of plants only convey 

 air, and these occur only in such plants where the close union 

 of thickened septa is said to take place by the so-called in- 

 tercellular substance. In last year's report this subject was 

 treated of at great lengthf, and I am convinced by recent ob- 

 servations still more decidedly, that the intercellular substance 

 is no distinct substance, but consists of the thickened su- 

 perposed cellular septa. I recommend for such observations 

 old leaf-petioles of species of Rheum and the old stems of cu- 

 curbitaceous plants. 



M. Morren regards the pleurenchymatous cells of the wood 

 as the route of the ascending crude nutritive sap, for which 

 some explanation may be necessary, as this denomination is 

 not yet generally adopted in Germany. I proposed the word 

 pleurenchyma to designate the tubes of the liber in 1830, 1 sub- 

 sequently discovered that the so-called ligneous cells possess 

 a similar structure with the tubes of the liber, and distin- 

 guished in the first part of my Vegetable Physiology short 

 pleurenchymatous cells and elongated pleurenchymatous cells; 

 the latter comprise the tubes of the liber, the former the lig- 

 neous cells, which in Germany are more frequently desig- 

 nated by the name of prosenchymatous cells, by which 1 

 understand the ligneous cells of the Conifer. M. Morren 

 has very correctly mentioned the fact, that the crude nu- 

 tritive sap ascends in the wood not only from below upwards, 

 but also passes in a horizontal direction and even obliquely 

 through the wood, exactly in the same way as the circulation 

 of the sap takes place from cell to cell in the diachyma of the 

 various organs. M. Morren is, however, a^so of opinion, that 

 these short pleurenchymatous cells are of the same origin and 



* Considerations sur le mouvement de la seve des Dicotyledones. Bullet, 

 de PAcad. de Scienc. de Bruxelles, 1837, p. 300. 



f Lond. and Edin. Philosophical Magazine, vol. xi. p. 435. 



