330 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 



of salts contained in the sap of these plants renders them capable of a resistance 

 which is almost greater than that afforded by mucilaginous materials and gum-resins. 



It must, however, be remarked here that not all green leaf- or stem-cells contain- 

 ing abundant water have the function of storing it up for a drj^ season, and that the 

 'aqueous cell-groups and strands adjoining the green tissue, especially the so-called 

 outer aqueous tissue, in very many cases, has another important function, viz. the 

 conducting of carbonic acid to places where it can be assimilated, but this will be 

 described in the next chapter. 



An extreme reduction of the leaf-surface, combined with a formation of green 

 transpiring tissue in the cortex of the stem, is also shown in another group of plants 

 known by the name of " Switch " plants. They are characterized by thin rod- 

 shaped stems and branches, while the cactiform plants, on the contrary, always have 

 their axes but little branched, and massive, thickened, fleshy and rigid stem-struc- 

 tures which are unaffected by the wind. The switch-plants may be subdivided into 

 those which are flexible, hollow, and only slightly branched — as, for example, the 

 horse-tails (Equisetum), reeds (Scirpus), rushes (Juncus), bog-rushes {Schvenus), and 

 several cy peruses (Cyperus); and into broom-like shrubs with rigid woody boughs 

 breaking up into innumerable branches and twigs. The former are distributed over 

 the whole world ; the latter are principally to be found in Australia and in districts 

 bordering on the Mediterranean Sea. In Australia it is chiefly Casuarinas and 

 some genera of Papilionacese and Santalacese {SphcGrolobium, Vi'niinaria, Lepto- 

 meria, Exocarpus) which take on this odd form, and some of them even attain to 

 the size of trees. In the Mediterranean flora isolated species and groups from the 

 families of Asparaginese, Polygalacese, and Resedaceae are seen with thin, stiff, rod- 

 shaped, leafless branches, which project stifily into the air with green cortex; but 

 again, most of these plants belong to the Papilionaceae and Santalacese. Several 

 switch-plants of the papilionaceous genera Retaraa, Genista, Cytisus, and Spartiuvi, 

 growing together, often cover wide tracts of country in densely-crowded masses, 

 and thus contribute not a little to the scenic peculiarity of the district. Many small 

 rocky islands off the coast of Istria are entirely overgrown by Spartium scoparium, 

 which is represented in the illustration opposite. In May large golden flowers, 

 scented like acacias, appear on the green rods of the Broom, and then for a short 

 time the dark green of the switch-plant is changed into a brilliant yellow. On 

 passing near the coast, just at this time, the remarkable phenomenon is seen of 

 golden yellow islands rising above the dark blue sea. This floral adornment is, 

 however, but transitory, and nothing more monotonous and desolate than such a 

 dry unwatered islet, covered with these shrubs, can be imagined 



The SpartiuTn belongs to those switch-plants which are not entirely leafless, but 

 which develop little green lancet-shaped leaves at intervals on their long twigs. But 

 these are of such secondary importance that their green tissue can only form the 

 smallest portion of the organic substances necessary to the further growth of the 

 plants, and this duty chiefly falls to the share of the cortex of the switch-like 

 branches. The cortex is also characteristically formed in accordance with this fact. 



