THE PRINCIPAL TISSUES. 



71 



(a) Collenchyma may be studied in the stems, petioles, and leaf-ribs 

 of herbaceous Dicotyledons e.g., in species of SilpMum, Rheum, 

 Rumtx, Ghenopodium in many Ldbiatm, Solanacece, Begoniacece, Cu- 

 curbitacece, and many others; also in the petioles of the water-lily 

 and young stems of the elder. 



(6) Upon soaking in water, or upon treatment with nitric or sulphu- 

 ric acid, the thickened angles become greatly swollen. 



Fig. 55. Longitudinal radial section of stem of Echinooystis lobata. fp, epidermis: 

 co, collenchyma; pa, parenchyma;/, a single wood fibre, marked with "crossed' 

 (i.e., twisted) pits ; sp, intercellular spaces. X 500. From a drawing by J. C. Arthur. 



(c) Upon treatment with Schultz's Solution the thickened angles are 

 colored light blue. 



(d) Upon slight warming in a solution of potash, and then treating 

 with a solution of iodine in potassium iodide, the thickened angles be- 

 come colored dark blue. 



101. Sclerenchyma. In many plants the hard parts are 

 composed of cells whose walls are thickened, often to a very 



