146 



A TEXTBOOK OF BOTANY [Cn. IV, 6 



The water-conducting vessels are of two sorts, first, 

 elongated single cells, called TRACHEIDS, and second, tubes, 

 called DUCTS or TRACHEA (Fig. 101), which are formed from 



many cells of which the 

 intermediate walls have 

 been absorbed. Trache- 

 ids occur often intermin- 

 gled with ducts; they 

 form the ends of the 

 xylem part of the veins 

 in leaves, and they make 

 up wholly the secondary 

 growth of Pines and other 

 coniferous woods (Figs. 

 102-4). Ducts develop 

 usually from a single row 

 of cylindrical cells by 

 absorption of the inter- 

 mediate walls ; but some- 

 times many rows of cells 

 are involved, in which 

 case the duct becomes 

 large and visible to the 

 eye, as in Oak and some 

 vines, the single-row type 

 being usually invisible 

 without a lens. Though 

 tubular in structure, 

 ducts are never unlim- 

 ited in length ; many are 

 not more than a few 

 inches, few exceed a few feet, and the longest, which occur 

 in some vines, are only a few yards in length. In all known 

 cases, however, the ends of ducts and tracheids are in con- 

 tact with others of like sort, and the intermediate walls are 

 so constructed, with guarded thin areas, as to permit a ready 



FIG. 101. Generalized drawings of 

 typical tracheal elements ; highly magni- 

 fied. From left to right, a fiber-tracheid ; 

 pitted and spiral tracheids; spiral and 

 pitted ducts, which show end walls and 

 remnants thereof. (From Strasburger.) 



