MARKINGS OK DUCTS. 



85 



but, according to Mobl, from loft to right in a few plants. Thus 

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

 they run from left to right in the ducts first formed, but in the 

 reverse direction in those which are produced later. And by 

 interruption of the spiral it may have two directions in the 

 same duct, as in those of Cucurbita. 1 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 together ; but if the organ under- 

 goes elongation after the 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 loosely wound 

 spiral vessels are usually found in the posterior first-formed por- 

 tion of the vascular bundle nearest to the pith, while those lying 

 nearest the bark have close convolutions." 2 



273. Annular 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 nearly 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 youngest 

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



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

 Ringgefasse. 



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



Fio. 64. Vertical radial section of hypocotyl of Ricinus communis, illustrating differ- 

 ent markings of ducts; t' t, pitted; I, scalariform ; s' s, spiral, the spirals beginning to 

 uncoil. (Sachs.) 



