i86 



BOTANY AND PHARMACOGNOSY. 



Besides these forms of parenchyma there are some special 

 kinds which may be mentioned, as the somewhat branching cells 

 in leaves, and in the stems of various marsh plants, as in species 

 of Juncus and Pontederia. In calamus, large intercellular spaces 

 are formed, and these may be mistaken for the cells themselves 

 (Fig. lor, B). 



Mechanical cells or stereome include all those cells which 

 serve to keep the various parts of the plant in their proper posi- 

 tions one with reference to the other, and which enable it to with- 

 stand undue strain and pressure. There are two principal forms 

 of mechanical cells, namely, (a) collenchyma and (b) scleren- 

 chyma. 



s 



Fig. I02. Forms of ducts. A. — ^^Longitudinal section of stem of Cucurbita Pepo 

 showing various forms of ducts: A, annular; S, spiral; D, double spiral; C. close annular; 

 R, reticulate. B. — Ducts of glycyrrhiza rhizome: W, wall; B, bordered pores; P. oblique 

 simple pores. 



The Collenchyma cells are long cells (occasionally 2 mm. 

 long) which are thickened at the angles (Fig. loi, C). The 

 walls consist of cellulose, or a modification of it, known as coUen- 

 chym, and have a silvery or grayish-blue luster. Besides proto- 

 plasm these cells occasionally contain starch. They are found di- 

 rectly beneath the epidermis of herbaceous stems, petioles, and the 

 mid-rib of various leaves, and are particularly noticeable at the 

 angles of all stems that are ribbed or angled in transverse section. 



Sclerenchyma cells include all of those cells which have 

 more or less uniformly thickened walls composed of lignocellu- 

 lose and permeated by simple or branching pores. They have a 

 thin layer of protoplasm and relatively large vacuoles, which con- 

 tain tannin or tannin-like masses, and occasionally calcium oxal- 

 ate crystals or starch, and in dead cells the lumen or cell-cavity 



