THE PRINCIPAL TISSUES. 89 



posite in those formed later ; while in interrupted spirals 

 both directions occur in the same vessel. Ringed and reticu- 

 lated vessels are opposite modifications of the spiral form : 

 A 



Fig. 73. Scalariform vessels of the rhizoma of Pteris aqtiilina. A, longitudinal sec- 

 tion of an end (about one third of the whole) of a short vessel ;/, the fusiform ex- 

 tremity, with long pits placed transversely; B, a small portion of A, taken from x, 

 and much more highly magnified ; C\ a longitudinal section of a portion of the side 

 wall between two vessels : D, a similar section through the inclined end wall (A.f) ; 

 in the upper-part of D, at/ the wall between the thickening ridges is broken through. 

 A, X 142 ; the others X 375.-Af ter De Bary. 



the first are due to an under-development of the thickening 

 forces in the young vessels, resulting in the production here 

 and there of isolated rings (Fig. 72, v) ; reticulated vessels 

 are due, on the contrary, to an over-development, which 



