TIIE PRINCIPAL TIMUE8. 71 



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

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

 Rums.x, Chenopodium in many Labiate, txdanacece, Begoniaceoe, Gu. 

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

 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. 



Fie. 55. Longitudinal radial section of stem of EcJiittocystit tobala. ep, epidermic ; 

 oo, collenchvma ; pa, parenchyma : /, a single wood fibre, marked with " crossed " 

 (i.e., twisted) piu> ; tji, intercellular spaces, x 500. Prom a drawing by J. C. Arthur. 



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

 colored light blue. 



(d) Upon slight warming in a solution of ix>tash, and then treaties 

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

 come colored dark blue. 



1O1. Sclerenchyma. In many plants the hard parts are 

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



