17 



purport as the other cells, and are produced as it were .from 

 spheroid cells, the so-called merenchyma, and therefore the 

 fibrils of the root must also be considered as such cells which 

 take their origin in the root-stem, &c. Much may, however, be 

 objected to this notion ; the fibrils are exceedingly delicate pa- 

 renchymatous cells, and parenchyma and pleurenchyma differ, 

 not merely in the form of their cells, but also in their func- 

 tions. Pleurenchyma rather serves merely to conduct the saps; 

 the short pleurenchyma conveys the crude nutritive sap in its 

 ascent, the long pleurenchyma (tubes of the liber) conducts an 

 elaborated formative sap from above downwards. In the pa- 

 renchymatous cells the assimilation of the absorbed nutritive 

 substances takes place : they respire through the intercellular 

 passages, while in the pleurenchymatous cells no respiration 

 can exist on account of the want of intercellular passages. I 

 have enumerated in my Vegetable Physiology several cases 

 where intercellular ducts occur even between the tubes of the 

 liber, for instance, in Asclepiadece and Apocynece ; and that in 

 these cases the tubes at the same time convey a milky fluid, 

 which abounds in globules ! The origin of the radicular fibrils 

 from the parenchymatous cells may easily be observed. 



M. Morren next enters into a special consideration of an 

 inversely grafted branch of a Camellia) in order to show that 

 the direction in the ascent of the sap may not only be inverted 

 for some hours, but even for the whole lifetime of the plant. 

 The first, as is known, was long since scientifically proved by 

 Stephen Hales, and I have also endeavoured to show * that 

 the ascent of the sap in separated branches is caused entirely 

 by the transpiration of the leaves, &c., consequently the di- 

 rection of the sap may be varied at the will of the experimenter. 

 The ideas of ascent and descent in the motion of the crude 

 nutritive sap are evidently not altogether correct ; the crude 

 sap is always conducted to those parts of the plant which 

 are in want of it, and therefore it is also conveyed to the bud 

 of a cutting, even when this is grafted inversely on to the 

 stock. The vacuum which must arise from the transpiration 

 of the young bud and of its leaves is immediately filled with 

 the fluid which lies nearest to it, and in this way the crude 



* Pflanzen-Physiologie, vol. ii. Berlin, 1838, 

 C 



