32 



ELEMENTARY ORGANS OF PLANTS. 



32. As the vascular tissue in plants is only a modification 

 of the cellular, we find in it all those characters which we 

 have already described in the cells of the cellular tissue. 

 Vascular tissue sometimes assumes the appearance of dotted 

 or pitted duots^ which are continuous tubes of very conspicu- 

 ous caliber or bore, which have been formed out of a series of 

 porous cells, placed end to end, the walls of which have been 

 absorbed in the progress of growth. The open mouths of 

 these ducts are conspicuous to the naked eye on the cross sec- 

 tion of the stems of the oak, poplar, and willow, rendering 

 apparent the limits of each year's growth in" the successive 

 rings of the stem. The tubular tissue, formed by porous ves- 

 sels, is termed Bothrenchyma ((3o9poj, a pit). Fig. 8 is a 

 portion of a dotted duct from the vine, evidently made up of 

 a short series of cells. Fig. 9 is part of a smaller dotted duct, 

 showing no appearance of such composition. 



33. When cells containing spiral deposits of sclerogen are 



Fig. 8. Fig. 9. 



placed end to end, and their parietes absorbed, then vascular 

 tissue takes the form of scalariform, annular, and spiral ducts, 



