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 Jiincus and Pontederia. In calamus, large intercellular spaces 

 are formed (Fig. iQi, B). 



The Mechanical Tissue comprises four types of cells : scle- 

 rotic, coUenchymatic, stereomatic and libriform. Of these the 

 libriform cells are scarcely to be distinguished from the stereo- 

 matic cells except by their position, being developed in the inner 

 part of the mestome-strands (or vascular bundle), inside the 

 cambium ring, hence the libriform cells accompany the vessels or 

 tracheae (Fig. 104, WF). 



B 



A 



1 ^_ ,^ ^: 'iX-.-^C; 





Fig. 102. 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 sclerotic cell is of the parenchymatic type but with very 

 thick, lignified walls having many layers and simple pores which 

 are spherical in surface sections. This type of cells contains only 

 air or an aqueous liquid, but never nutritive matters, as in Cocoa- 

 nut, Walnut shells. Vanilla (Fig. 313), poppy capsule (Fig. 314). 

 Sclerotic cells are also referred to as " Stone cells " (Figs. 301, 

 302). 



The Collenchyma cell is elongated, prismatic, with soft walls 

 consisting mainly of cellulose and never lignified ; the contents 

 being rich in water. Tn transverse section it is readily distin- 

 guished by the local thickening of the walls, i.e., at the angles of 

 the cells (Fig. loi, c). Pores are rare, but when present they 

 are annular or slit-like. Collenchyma occurs near the surface 

 of plant organs, as herbaceous stems, when they form ribs, as 



