7*2 THE MECHANICAL FUNCTIONS. 



the partitions which separate their cavities may 

 become obhterated at the points of junction, 

 so as to unite them into one continuous tube 

 with an uninterrupted interior passage. This 

 view of the formation of the vessels of plants is 

 confirmed by the gradation that may be traced 

 among these various kinds of structures. Elon- 

 gated cells are often met with applied to each 

 other endwise, as if preparatory to their coales- 

 cence into tubes. Sometimes the tapering ends 

 of fusiform 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 ap- 

 plication to each other in the same line, being 

 separated only by membranes passing trans- 

 versely ; 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 con- 

 tinuous, 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 monilifonn, jointed 

 or headed vessels.^ Traces of the membranous 

 partitions sometimes remain where their oblite- 

 ration has been only partial, leaving transverse 

 fibres. The conical terminations occasionally 



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



