TSB Pm:TCIPAL TISBUBS. 



83 



posite in those formed later; while in intermpted spirals 

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

 lated yessels are opposite modifications of the spiral form ; 

 A JB 



T'ig. 73. — Scalariforravessela of the rhizoma of J^eHs aquUina. ^, longitudinal sec- 

 tion of an end (abunt one third of the whole) of a short vessel ; /, tlie fnsiform ex- 

 tremity, with long pits placed transversely ; 5, a small portion of A, taken from x^ 

 and much more highly mjigtiified ; C, a longitudinal section of a portion of the side 

 wall between two 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 nnder-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, which 



