20 



ANATOMY AND PHYSIOLOGY OF 



Fig. 30. 







'INlll 



jiiiii"'' 



Mi 



iraiuiuiii'"'^' 



fllll 



jii until'""' 



h 



X" 





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longitudinal growtli, the turns of tlie spiral fibre lie close together (fig. 

 27) ; but if the organ undergoes elongation after the completion of the 

 development of the vessel, the turns of the fibre ai^e drawn far apart (figs. 

 28, 29), by the stretching which the vessel sufiers ; consequently, very 

 loosely wound spii^al vessels are usually found in the posterior first-formed 

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

 the bark have close convolutions. 



The annular vessds (fig. 30) form a slight modification of the vspiral 

 vessels, for in many cases a series of vascular utricles containing spiral 

 fibres are regularly found followed in the same vessel 

 by a seiies of utricles which contain annular fibres, or 

 spiral fibres and annular fibres alternate without any 

 definite rule, often in the same vessel 



The reticulated vessels occur in manifold modifications, 

 in particular among the vascular Cryptogamia, and in 

 tlie outer youngest parts of the vascular bundles of the 

 Monocotyledons. In these occurs a dependence of the 

 form and distribution of the pits upon the formation of 

 the adjacent parts, similar to that which we have found 

 in the pitted cells. "When several vessels lie immedi- 

 ately upon one another, the walls by which they are 

 coberent together {^g. 31, a) are covered with trans- 

 verse pits, separated by narrow fibres, and these pits 

 occupy the whole breadth of such a side-wall, but are 

 not continued over the angles at which the several 

 lateral faces of the vessel meet. To this form is applied 

 the term sccdarifofiin ducts. But if the wall of such a 

 vessel is in contact with cells by a large or small surface 

 ^^ ^fibre! ^^^^^ (fig- 3i, 5) its pits exhibit the elliptical or rounded form 

 of the pit of the cells, and are sometimes distributed 

 quite irregularly, sometimes arranged in a spiral direction, and the vessel 

 retains the name of reticulated. Yery frequently the same vessel exhibits 

 both these modifications of structure at different points. 



Lastly, the fitted vessels (fig. 32) which occur in the wood of Dico- 

 tyledons (with the exception of its 

 oldest parts, in contact with the pith) 

 exhibit on those points of their walls 

 by which they are in contact with a 

 second vessel, a more or less abundant 

 quantity of pits surrounded by a line, 

 while the walls borderiag on cells pre- 

 sent the form of reticulated vessels, 

 i. e,j possess pits without a boundary 

 line, or are quite devoid of them. In 

 some cases, for example in the Lime, a 

 tertiary membrane occurs in the pitted 

 vessels, which appears in the form of 

 fibres running between the pits. 



The septa between the vascular utri- 

 cles do not always become perfectly 

 absorbed ; but in the reticulated^ and especially often in the pitted vessels, 



Ves'=?el from the 



stem of a Gourd, 



contaming both 



Fiff, 32. 



Fig^ 31. 



a V 



Eeticulated ve^el from 

 a tree Fern. 



Pitted vessel firom 

 Laarus Saasafras 



