vegp:table organization. G.3 



of developement in one particnlar direction ; and 

 it is easy to conceive that where the extremities of 

 these elongated cells meet, the partitions which 

 separate their cavities may become obliterated at 

 the points of junction, so as to unite them into one 

 continuous tube with an uninterrupted interior pas- 

 sage. This view of the formation of the vessels of 

 plants is confirmed by the gradation which may be 

 traced among these various kinds of structures. 

 Elongated cells are often met with applied to each 

 other endwise, as if preparatory to their coalescence 

 into tubes. Sometimes the tapering ends of fusi- 

 form cells are joined laterally (as seen in Fig. 12), 

 so that the partitions which divide their cavities 

 are oblique. At other times their ends are broader, 

 and admit of their more direct application to each 

 other in the same line, being separated only by 

 membranes passing transversely ; in which case 

 they present, under the microscope, the appearance 

 of a necklace of beads (Fig. 13). When, by the 

 destruction of these partitions, their cavities become 

 continuous, the tubes they form exhibit a series of 

 contractions at certain intervals, marking their 

 origin from separate cells. In this state they have 

 received the names of moni lif or m, jointed or beaded 

 vessels* Traces of the membranous partitions some- 

 times remain where their obliteration has been only 

 partial, leaving transverse fibres. The conical ter- 

 minations occasionally observable in the vessels of 

 plants also indicate their cellular origin.! 



* Mirbel gave them the name of " Vaisseaux en chapelet." 

 t This theory of the derivation of vessels from cells was first 

 advanced by Treviranus, and has been fully confirmed by subse- 

 quent observers, and particularly by Schleiden. 



