PROSENCHYMATOUS TISSUE 



255 



them the protosplasm strands extend, as shown in the 

 figure. Their exact function is not known. 



414. PROSENCHYMATOUS TISSUE. Several elongated 

 and strong tissues, which greatly strengthen the stems in 

 which they are found, are 



collectively known as pros- 

 enchyma. The cells of 

 these tissues become much 

 thickened by the addition 

 of layers to the inner sur- 

 face, and finally lose their 

 protoplasm. They may, at 

 times, serve as store-rooms 

 for starch and other nu- 

 trients, and take an im- 

 portant part in the trans- 

 fer of the plant juices. 

 Some writers call this 

 group of tissues scleren- 

 chyma. 



415. There are four 

 main varieties of tissues 

 which may be included 

 under prosenchyma. (1) 

 Fibrous tissue, composed 

 of very thick -walled cells 

 with very small central 

 eavities. F, Fig. 401. 



They are very long and tapering at the ends, which lap. 

 Such tissue is found in many plants where it often 

 wholly or in part surrounds the fibro- vascular bundles. 

 It is more often but not always found near the soft 

 bast: hence the cells are sometimes called last filers or 

 hard bast. (2) Wood tissue, or wood fibers. This is 

 composed of cells much like the preceding in structure, 



397. Bast-tissue, s, s, sieve tubes; c, com- 

 panion cell ; p, shows a top view of a 

 sieve plate, with a companion cell, c, at 

 the side ; o, shows sieve plates in the 

 side of the cell. In s, s, the proto- 

 plasm is shrunken from the walls by 

 reagents. 



