CHAPTER VII 

 THE FIBROVASCULAR TISSUES: SECONDARY WOOD VESSELS 



The vessel is an element of structure which in the higher forms 

 and in the secondary wood is of extremely great evolutionary 

 importance. Before we take up this type of ligneous element in 

 the seed plants it will be well to consider its occurrence in lower 

 forms and in the primary wood. It was pointed out by De Bary 

 many years ago that many of the scalariform tracheids of the 

 bundles in the bracken fern are of the nature of vessels, using that 

 term to apply to elements which are not merely pitted but actually 

 perforated at the ends, thus permitting a much more ready passage 

 of water. The statements of the distinguished anatomist of 

 Strassburg have been confirmed and denied by more recent inves- 

 tigators, but there seems to be no doubt whatever on the basis 

 of our improved technique that he was in every respect correct. 

 Fig. 'jia illustrates the organization of a vessel in Pteris aquilina 

 isolated from the surrounding tracheids by maceration. It is 

 clear that the scalariform bars are separated from one another by 

 wider intervals at the ends of the element and that the bars them- 

 selves are considerably more slender in this region and are not 

 distinguished by as pronounced overhanging margins as is the case 

 in the similar structures on the lateral walls. In b is represented 

 a profile view of the terminal inclined wall of the tracheid, indicat- 

 ing plainly the absence of the membrane which characterized the 

 lateral scalariform pits. Further, the margin or border in the 

 case of the terminal open pits is slightly developed in comparison 

 with that of the lateral pores with membranes intact. Conditions 

 simila/ to those found in the bracken appear in many other repre- 

 sentatives of the Filicales and are by no means confined to the 

 Polypodiaceae, which at the present time not only are the largest, 

 but also are considered in many respects the most specialized family 

 of the group. Elements resembling vessels in the presence of 

 terminal perforations have likewise been described for the genus 



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