THE PRISrCIPAL TISSUES. 



83 



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 



Pig. 73.— Scalariform vessels of the rhizoma of Pteris aqiiUina. A, longitudinal sec- 

 "tion of an end (abont on« third of the whole) of a short vessel : /, the fusiform ex- 

 tremity, with long pits placed transversely: B, a small porti(m of ^, taken from a;, 

 and much more highly miigiiihed ; (7, a longitudinal section of a portion of the side 

 ^valI between t\vo vessels ; JJ, a similar section through the inclined end wall {A,f) ; 

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

 A, X 142 ; the others X 375. —After 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. 73, v) ; reticulated vessels 

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



