512 PHYSIOLOGY. 



long and the course of the vessels straight ; in roots, and in concentrated 

 rhizomes and conns, &c., the constituent cells are mostly short and the 

 course of vessels tortuous. The spiral fibre in the interior of these vessels 

 has been considered to be hollow or tubular : but this is not generally 

 regarded as correct. Ultimately they are empty alike of protoplasm and 

 cell-sap, and serve as air-conductors. 



Annular vessels (tig. 560) are found in situations similar to the last, 

 being generally formed a little later in the same bundles. They are com- 

 monly of greater diameter than true spirals. This is the commonest form 

 of vessel in the Equisetacese. 



Reticulated vessels (fig. 562) are abundantly developed with the spiral 

 and annular kinds in succulent stems, roots, petioles, &c. They are very 

 important constituents in the fibro-vascular bundles of Monocotyledons 

 generally. They are mostly of rather large diameter ; their cells long in 

 stem-structures, short and irregularly formed in roots and in the inner 

 cortical region of Monocotyledonous stems, where a number of vessels are 

 often anastomosed into a kind of network. 



Scalariform vessels (fig. 541) are especially characteristic of the woody 

 structures of the Ferns and Lycopodiacese, in which they sometimes occur 

 of very large diameter. Most vessels are cylindrical, and present a more 

 or less circular section ; but the scalariform are prismatic, usually with an 

 hexagonal section. 



Vessels, when once formed, are usually persistent; but in some water- 

 plants the stem when young is traversed by a single spiral vessel, which 

 disappears as the stem grows older, so that in the adult condition the 

 stem seems wholly cellular with a central lacuna. 



The pitted or dotted ducts (fig. 536) are characteristic of the wood of 

 Dicotyledons, where they occur either scattered in the prosenchyrna, or 

 forming the principal constituent of the wood. 



The walls of pitted ducts are not always uniform, this depending in 

 some cases upon the nature of the organs with which they are in contact, 

 whether cells or other ducts, since the pits always correspond on the walls 

 of adjacent organs, and they are ordinarily less numerous and less regular 

 on the walls of prosenchymatous cells than on those of ducts. 



The pits and their borders (p. 487) are very generally somewhat elon- 

 gated obliquely ; and the canal of the pit is often enlarged into a trans- 

 verse slit in the inner part, which in some cases becomes confluent with 

 that of its neighbours. In some plants we find ducts with the wall 

 marked both with pits and a spiral fibre, like the walls of the wood-cells 

 of Taxus (fig. 542). 



Pitted ducts with uniform walls make up the chief mass of the wood 

 of Clematis. In the wood of Elder, Beech, Hazel, Alder, &c. we find ducts 

 with pits numerous on the walls adjoining other ducts, but distant or 

 absent on the walls adjoining wood-cells. In Bombax the wood-cells are 

 for the most part replaced by parenchyma-cells, and the walls of ducts 

 adjoining these have the pits destitute of the border &c. 



Pitted ducts form the large tubes, visible to the naked eye, seen in cross 

 sections of most woods, especially Oak, Mahogany, &c. They are absent 

 from the wood of the Coniferse, which is wholly composed of simple 

 wood-cells (fig. 537). 



Vasa propria are elongated cells with thin walls, and either obh'que 



