12 



SCIENCE PRIMERS. 



[in. 



wood is principally formed, consists of long cells, or 

 rather tubes, tapering and closed at both ends, with 

 thick walls, and which lie side by side and form wood. 



8. Bast-tissue consist of very long flexible cells, 

 or rather tubes, also closed at both ends. It occurs 

 chiefly in the inner bark, and supplies the materials of 

 many useful fabrics. Hemp and flax are bast-cells 

 of the plants of those names ; and the Bast, used by 

 gardeners for tying, is the inner bark of the lime-tree. 



9. Vascular tissue consists of long, unbranched 

 tubes, with thin walls, which are often dotted or 



FIG. 4. Spiral-vessels with cellular tissue on each side, many times the 

 real size. 



barred, and sometimes thickened internally by spiral 

 threads, easily seen in the leaf of the hyacinth, if 

 broken across. These are called spiral vessels (Fig. 

 4). All such tubes are formed from rows of super- 

 posed cells, the partitions which separate them having 

 been absorbed. 



The tissues 7, 8, and 9 usually occur together in the 

 form of bundles which traverse the cellular tissue, as 

 the veins (or nerves) of the leaves, and are called 

 fibre-vascular bundles. 



