THE PRINCIPAL TISSUES. 



71 



(fls) Collenchyma may be studied in tlie stems, petioles, and leaf-ribs 

 •of herbaceous Dicotyledons — e.g., in species of Silphium, Rheum, 

 Rum'x, Olienopodium — in many Labiate,, Solanaoem, Begoniaaem, Cu- 

 <,urbitaeem, 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. 



Kg. 55.— Longitudinal radial section of stem of Echinooijstis lobata. ep, epidermis : 

 eo, collencliyma ; pa, parenchyma ; f, 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 

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



