FIBRO VASCULAR TISSUES: VESSELS 



93 



Selagimlla among the lycopodineous forms. It is thus clear that, 

 so far as the primary structures of the wood are concerned, vessels 

 are present even in the case of 

 representatives of the lower vas- 

 cular plants. 



Although in certain ferns 

 and lycopods structures occur 

 which, physiologically at any 

 rate, represent vessels, these 

 cannot be regarded as quite on 

 the same morphological and 

 evolutionary footing as the ves- 

 sels of the highest gymnosperms 

 and the angiosperms. The step 

 from a tracheid to a vessel, where 

 all the tracheids are scalariform, 

 as is the case with the tracheids 

 of ferns and lycopods, is a much 

 shorter one than when the 

 fibrous elements are pitted and 

 not scalariform. We find as a 

 consequence that, although ves- 

 sels or elements which have been 

 regarded as such occur low in 

 the scale of the Vasculares in 

 the primary structures, they 

 make their appearance in the 

 secondary wood only in the 

 higher representatives of the 

 seed plants. 



The secondary wood of the 

 extinct Paleozoic arboreal cryp- 

 togams, although often perfectly 

 preserved, in no authentic 

 instance has yet revealed ele- 

 ments which may with any degree of propriety be designated as 

 vessels. The same situation obtains in the case of the lower 



FIG. 71. -Face and profile views of 

 the type of vessel found in the primary 

 wood of the bracken. 



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