78 FORMATION OF VESSELS [CH. VI 



this preliminary sketch of the nature of cells, that in 

 definite situations in the tissues of the higher plants 

 with which alone we are here concerned long series of 

 cells become converted into thin capillary tubes or pipes, 

 called vessels, by the perforation of their separating walls 

 so that their cavities become continuous end to end ; by 

 the thickening of their cell- walls to give rigidity to the 

 side-walls of these pipes and so enable them to withstand 

 pressures and strains of various kinds ; and by changes in 

 their contents,* often resulting in the entire disappearance 

 of the protoplasm, nucleus, &c, so that their cavities fill 

 with water and gases of various kinds, or with liquids of 

 special nature in different cases. We are not here con- 

 cerned with the details of structure, size, or the many 

 peculiarities of marking on these vessels ; it is sufficient 

 for our present purpose that these pipes convey fluids 

 more or less rapidly from place to place in the stem, roots, 

 leaves and other organs, to and from the cells in which 

 such fluids are used or prepared, and in which they 

 could only move slowly by diffusion through the separating 

 cell-walls. 



