146 



A TEXTBOOK OF BOTANY 



[Ch. IV, 6 



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

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

 called ducts or tracheae (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 j 

 tubular in structure, 

 ducts are never unlim- 

 ited in length ; many are 

 not more than a few I 

 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.) 



