482 MECHANISMS FOR CONVEYANCE TO AND FRO. 



passed through the dividing wall, a solidification, occurs in each of the succeeding 

 members of a string of cells. This is a very laborious and wearisome process, and 

 the question involuntarily arises, after observing these methods of transmission, 

 why these numerous partition walls in the rows of cells are not done away with. 

 The wood vessels have been produced from rows of cells by the solution of the 

 dividing partition walls; why are the many transverse walls retained here to 

 complicate and retard the transportation of the substances? It must be supposed 

 that these cross walls, which break up the free channel, are in some way beneficial 

 to the plant, since they occur so generally and with such regularity. It might be 

 thought, first of all, that these walls keep open the road, and that thereby the 

 delicate walls of the cells forming the channel are protected fi'om collapse. Apart 

 from the fact that the cells of bast parenchyma, imbedded in niches and grooves in 

 the periphery of the hard wood, are prevented from collapsing by their sheltered 

 position and nevertheless exhibit transverse walls, while the delicate-walled 

 laticiferous tubes, which are anything but well-protected, possess none and yet do 

 not collapse — apart from this, such a delicate wall would form but a bad stiffening 

 agent, and the support would be obtained much better by band-like circular 

 thickenings. It has also been surmised that the cross walls inserted in the channels 

 might be of use in that they prevent an excessive accumulation of solid bodies at 

 certain places on the road. Where the cells of a cell-row^ stand vertically above one 

 another, as, for example, in erect stems, it is found that the small starch-granules 

 sink to the bottom of the cells and lie on the lower transverse wall. If all the 

 solid corpuscles contained in the sap of a long vertical tube were to sink to the 

 bottom, of course an obstruction might arise which would be anything but 

 beneficial. But the significance of the partition walls most probably lies in the 

 transformations they produce in the substances. It may be safely assumed that 

 those materials which must pass through not merely the cellulose transverse wall, 

 but also the protoplasmic parietal layer of the cell-chamber, undergo an alteration 

 thus under the influence of the living protoplasm; that the position of the atoms 

 becomes different, or that new atoms enter into combination and others are 

 displaced, in short, that re-arrangements and transformations occur from which it 

 results that the transmitted materials arrive at their destinations prepared in the 

 best possible way. With this, however, we return to the important theorem 

 previously stated, that these rows of cells have not merely the significance of a road 

 along which the materials, formed at the starting-points, are conducted unchanged 

 to the terminal stations; but that they also form places for the continuous ti'ans- 

 formation and alteration of these materials for subsequent use. 



