PLANT TISSUES 



walls of these tubes are of varying thickness, usually, however, 

 thinner than those of woody fibers. The thickness is due to an infil- 



d b 



c d b 



B 



FIG. 49. 





o v o 





y$fi 



OOoC 

 00 



6 C 



FIG. 50. 



FIG. 49. Stages in the development of sieve tubes, companion cells, and 

 phloem parenchyma. A, a. and b, Two rows of plerome cells; in c and d, a has 

 divided longitudinally and c is to become companion cells; d, a sieve tube, and 

 b, phloem parenchyma. B, c, Companion cells, and d, a beginning sieve tube 

 from c and d, respectively in>4. The cross- walls in d are pitted; b, phloem paren- 

 chyma grown larger than in A. C, The same as B with the pits in the cross- 

 walls of the sieve tubes become perforations, and the nuclei gone from the cells 

 composing the tube. (From Stevens.) 



FIG. 50. Vascular elements. A, annular tracheal tube; B, spiral trachea 

 tube; C, reticulated tracheal tube; D, pitted tracheal tube; E, cross-section 

 through plate of seive tube, and adjoining companion cell; F, length- wise section 

 of sieve tube; G, portions of two companion cells. (A, B, C, D, Robbins; E, F, 

 and G, after Strasburger.) 



tration of lignin upon the original cellulose wall, 

 characteristic thickenings on their inner surfaces. 



8 



The walls show 



