THE CELL. 35 



rows. They are lifeless because they no longer possess a primordial 

 utricle. It is these vessels that are characterized by the thickenings 

 under discussion. 



The designation annulay^ (ring) and spiral vessels for a and J, 

 and reticulated, scalariform^ and porous (pitted), for c, tZ, and 6', need 

 no further explanation. The five forms represent mechanically sup- 

 ported tubes serving the purpose of conducting water in the plant- 

 body. "Why mechanically supported ? Because they are contigu- 

 ous with living cells, and these living cells are capable of exerting a- 

 high hydrostatic pressure, and actually force water through thin por- 

 tions of the cell- walls into the vessels; otherwise they would be com- 

 pressed by the living cells, since they are not capable of exerting any 

 active pressure themselves. Such is the purpose of the thickenings 

 described. Further, it is known that vessels contain alternately air 

 and water, and that when branches are cut under mercury or colored 

 liquids the vessels become filled to a given height with these liquids. 

 (This is also true of mercury in spite of its capillary depression.) 

 Therefore there is at times a negative pressure within the vessels, 

 •which again necessitates mechanical support. The structural rela- 

 tions are thus teleologically explained. Annular and spiral' vessels 

 on the one hand, and reticulate, scalar! form, and porous (pitted) 

 vessels on the other, differ very materially in one respect. This 

 difference may be indicated by the designation primary {a and h) 

 and secondary (r?, 6?, e). The following will serve to explain it. 



In general only thin portions of cell-walls are capable of growth. 

 From this it follows that annular elements are given considerable 

 scope for growth in length by the localized surface-growth between 

 the rings thus forcing them farther apart. Also the spirally thick- 

 ened elements, whose spirals are at first closely wound, grow at the 

 thinner portions, causing the spirals to become more slanting. An- 

 nular and spiral vessels stretch in a longitudinal direction. Such 

 growth of thin-walled portions, accompanied by elongation of the 

 entire element, cannot take place in the vessel-forms c, d^ and e be- 

 cause of the firmly adhering longitudinal thickenings. This again 

 explains from a teleological standpoint why a plant-organ contains or 

 develops annular and spiral vascular elements during the period of 

 growth, and that during the second period, that is, when growth in 

 length has ceased, the secondary non-elongating vessels are more 



' The German expression " Schraubengefdss " (screw-vessel) is more correct. — • 

 Tkans. 



