166 CENTRIC LEAVES 



around the whole periphery, but in surface sections are seen 

 to be arranged in approximately longitudinal series. Beneath 

 the epidermis is a typical palisade tissue (C.) of two to four layers 

 of cells, interrupted by large gaps (the respiratory cavities) below 

 the stomata. Within the assimilatory tissue follows an irregular 

 ring of typical Monocotyledonous bundles (V.b.) accompanied by 

 sclerenchyma. The large central cavity (p-c.), which is bounded 

 by parenchyma (p.), is interrupted at intervals by transverse 

 septa containing vascular elements. The Jointed Rush affords 

 an excellent example of those plants which, though growing in 

 wet places, exhibit a structure that suggests a need for economy 

 of water (cf. p. 177). The reason is not yet fully understood. 



A less extreme type of centric structure is found in Hakea, 

 a native of Australia, in which the leaves are pinnately branched, 

 the cylindrical segments being slightly flattened on the upper 

 surface. A transverse section of a pinna shows the customary 

 thick cuticle and deeply sunk stomata. Beneath the epidermis 

 are two layers of palisade cells* which completely encircle the 

 central parenchyma containing three prominent vascular bundles. 

 Stretched between the epidermis and this central region 

 are occasional mechanical elements having thick whitish walls 

 and slightly dilated ends, and probably serving to prevent collapse 

 of the palisade tissue during periods of drought. The vascular 

 bundles are accompanied, both on their upper and lower sides, 

 by strands of sclerenchyma. The central region "in which the 

 bundles are embedded consists of colourless water-storing 

 (aqueous} tissue in which are scattered occasional large tannin- 

 cells having deep brown contents. A similar type of centric 

 leaf is seen in the Scotch Fir (Pinus, cf. p. 343). 



Aqueous tissue, already observed in the Butcher's Broom 

 (Fig. 80, Aq.} and Hakea, is a most prominent feature of succu- 

 lents. In the leaves of the Stonecrop (Sedum), the Prickly 

 Saltwort (Salsola kali), and the Sea-blite (Suceda), the large 

 colourless and thin-walled cells in which water is stored up form 

 the bulk of the leaf -tissue. The aqueous tissue occupies the 

 centre of the leaf, with the assimilatory tissue towards the 

 periphery ; in the Prickly Saltwort these tissues are sharply marked 

 off from one another (cf. also Fig. 44, A, p. 94) , but in the other two 

 cases there is quite a gradual transition between them, Similar 



