MAKKIXGS OF DUCTS. 



85 



but, according to Moli], from left to right in a few plants. Thus 

 in the wood of Vitis vinifera, Berberis vulgaris, and some others, 

 the}- run from left to right in the duuts first formed, Ijut in the 

 reverse direction in those whicli are produced later. And by 

 interruption of tlie spiral it maj- have two directions in the 

 same duct, as in those of Cucurbita.^ The steepness of the 



spiral depends in part on the age of the cell, or vessel, — at least 

 in some cases. According to Mohl, " if the vessel is developed 

 in an organ which has already completed its longitudinal growth, 

 the turns of the spiral lie close togetiier ; but if tlie organ under- 

 goes elongation after tlie completion of the development of the 

 vessel, the turns of the fibre are drawn far apart by the stretch- 

 ing which the vessel suffers ; consequently very looselj- wound 

 si)iral vessels are usuallj- found in tlie posterior first-formed por- 

 tion of the vascular bundle nearest to the pith, while those Ij'ing 

 nearest the barlt have close eonvohitions."^ 



273. Anmilar and reticulated markings have been regarded as 

 mechanical modifications of spirals, and it is true that inter- 

 mediate forms exist between these types. For instance, tightly 

 wound spirals are nearlj' annular, and in some cases there are 

 threads which run either vertically or obliquely from one part of 

 a spiral to the contiguous thread. But even in the 3-oungest 

 states of some ducts the markings appear as rings or as a net- 



1 Mohl : Vermischte Scliriften, 1845, pp. 287, 321, Ueber den Ban der 

 Eingsefasse. 



2 Mohl : Vegetable Cell, Eng. Trans., 1852, p. 19. 



Fig. 64. Vertifal radial section of liypocotyl of Ricinus communis, illustrating differ- 

 ent markings of ducts j i' /, pitted; I, scalariform; s' s, spiral, tlie spirals beginning to 

 uncoil. (SacUs.) 



