68 THE VEGETATIVE ORGANS 



secretion, e. g. Campion and Catchfly ; while in other 

 plants they secrete a poisonous acid and the hairs become 

 formidable stings, e. g. Nettle. 



The cortex and mechanical supporting tissues. Beneath 

 the epidermis is the cortex, and the cells of the outer part, 

 especially in herbaceous stems, may be strongly thickened 

 so as to form a firm supporting tube (Fig. 36 sc l ). In the 

 Deadnettle, the cortical cells of the angles of the stem are 

 strongly thickened at their corners (Fig. 34), and these, 

 together with the wood of the large bundles, are effective in 

 protecting the stem against the stresses of tension and com- 

 pression ; in fact the whole stem is built on the principle 

 of a box girder. A tissue consisting of cells which retain 

 their living contents, and whose walls are thickened at the 

 corners, is called collenchyma(Gr.fo//g = gelatinous matter, 

 chyma = an infusion). A ring of collenchyma occurs in 

 the outer cortex of the Sunflower stem (Fig. 37), and it 

 is frequently found in leaf-stalks. Bands of cells in the 

 cortex of the Bracken and Birthwort are uniformly thickened 

 and devoid of living contents. Those of the Birthwort 

 (Fig. 36) form a strengthening ring immediately beneath 

 the epidermis (sc 1 ), and a second one (sc 2 ) as a strengthen- 

 ing cylinder midway between the epidermis and the bast. 

 Such a mechanical supporting tissue is called sclerenchyma 

 (Gr. skleros = hard). 



The ring of wood in the Elder (Fig. 38, w) forms a sup- 

 porting mechanism on the principle of a hollow pillar, while 

 in older woody stems the arrangement is that of a solid 

 pillar. Woody tissues thus perform a double function 

 conduction of crude sap and mechanical support. Many 

 ingenious devices for mechanical support may be found in 

 the stems of plants, and several common species should be 

 compared in this respect. 



Structure and arrangement of vascular bundles. A com- 

 parison of Figs. 31 to 34 and 36 to 39, which are transverse 



