158 



PHYSIOLOGICAL BOTANY. 



404. Annular or ringed ducts are produced by the 

 union end to end of annular cells, the walls of which 

 are held apart by rings or hoop-like 



thickenings on the inner surface (Fig. 

 507, A, B, 0). 



405. Scalariform ducts, character- 

 istic of ferns, are formed when the an- 

 nular vessels are compressed into pris- 

 matic forms whose sides present the 

 appearance of ladders (Fig. 507, D). 



406. Dotted or pitted ducts are 

 formed of dotted or pitted cells, as in 

 the case of annular cells (Fig. 507, E). 



407. Sieve ducts or tubes are 

 formed of colorless elongated cells, of 



large diameter; the walls are soft and from Mamillaria - 



very much enlarged and thickened at the joints ; at 



the junctions finely perforated plates appear (Fig. 



506 



50 i 



507, A, annular vessel from leaf-stalk of Melon ; B, duct, part spiral and part annular, from Melon ; C, 

 part annular and part reticulated duct from Melon ; D, annular and reticulated ducts pressed into pris- 

 matic shape, forming scalariform vessels, from Tree-fern ; E, pitted or dotted duct, formed by the union 

 of pitted cells from melon-stalk. 



508, A) ; also on the internodes are spots of fine per- 

 forations and slits (Fig. 508, B). These spots of per- 

 forations are like a strainer or sieve ; hence the name. 



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