PROSENCHYMA 



281 



At times they may serve as store-rooms for starch and 

 other products, and take an important part in the transfer 

 of the plant juices. 



478. There are four main varieties of tissues that may be 

 included under prosen- 

 chyma. (1) Fibrous tissue, 

 composed of very thick- 

 walled cells with very 

 small central cavities. (F, 

 Fig. 454.) They are very- 

 long and tapering at the 

 ends, which lap. Such 

 tissue is found in many 

 plants where it often 

 wholly or hi part surrounds 

 the fibro-vascular bundles. 

 It is more often but not 

 always found near the soft 

 bast: hence the cells are 

 sometimes called bast 

 fibers or hard bast. (2) 

 Wood tissue, or wood fibers. 

 This is composed of cells 

 much like the preceding 

 structure, but with 



in 



450. Bast-tissue. , s, sieve tubes; e, 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 cek. 

 In s, s, the protoplasm is shrunken from the 

 walls by reagents. 



thinner walls and the cen- 

 tral cavity not so nearly 

 closed. In some cases 

 such fibers have transverse walls. Wood cells constitute 

 a large part of the wood of some plants and in other 

 cases are scattered only among the other prosenchyma. 

 (3) Tracheids. Cells of this tissue differ from ordinary 

 cells in being supplied with numerous bordered pits or 

 other characteristic markings. They constitute almost all 

 of the wood of the pines and other gymnosperms. (Fig. 



