24 COLLEGE BOTAtfY 



in others it serves for the storage of reserve foods. The parts of 

 the plants that we use for food are made up almost entirely of 

 parenchyma cells. 



Sclerenchyma (Fig. 7). This tissue is derived from the 

 parenchyma and is composed of cells with thickened walls, which 

 are frequently referred to as hard or stony cells. They are usu- 

 ally irregular or polygonal in shape, but may be elongated or 

 even branched. The walls are thick, lamellated and porous. 

 The pores may be simple or branched. The cell cavity is usually 

 small and during the early life of the cell is lined by a thin 

 layer of protoplasm. These cells frequently contain tannin, cal- 

 cium oxalate crystals, starch and other substances. The shells 

 of nuts are made up almost entirely of these cells. This tissue 

 is also found in varying amounts in roots, stems, leaves and 

 fruits of various plants. It serves primarily for the protection 

 of delicate and nutritious parts against animals that would use 

 them for food and against the elements of nature that would 

 destroy them. The fibrous cells are sometimes sclerenchymatized. 



Prosenchyma or Fibrous Tissue. This tissue is derived 

 from the parenchyma (Fig. 7) . The cells are usually very much 

 elongated, sometimes being more than an inch in length and 

 interlaced or dove-tailed so as to form very strong structures. 

 They are found in all parts of the plant, but are especially abun- 

 dant in the stems and roots. They show so many modifications 

 that it is necessary to subdivide them into smaller groups : bast 

 fibers and wood fibers. 



(1) The bast fibers are long cells with thick walls. The cell 

 walls may remain cellulose or become very much lignified. They 

 are most abundant in the inner layers of the bark and give both 

 strength and flexibility to the stem. In the herbaceous plants 



