CENTRIC LEAVES 



165 



tective hairs can be gauged by comparing the rates of evaporation 

 of water from three similar wide-mouthed bottles (about 8 oz. 

 capacity) , the neck of one being protected by a cardboard collar 

 about i|- inches high, and that of a second by a similar collar 

 filled with thistle-down or cotton- wool. Each bottle should be 

 filled originally with the same volume of water, and this should 

 again be measured at the end of the experiment. After a few 

 days' exposure in the open air, preferably when it is windy, it 

 will be found 

 that the bottle 

 with thistle- 

 down has lost 

 least water, 

 whilst that 

 unprotected 

 by a collar 

 has lost most. 

 There are, 

 however, 

 many other 

 peculiarities 

 that are asso- 

 ciated with 

 plants exhibit- 

 ing a struc- 

 ture that tends 

 to reduce 

 transpiration. 

 A f e atur e 



which often accompanies a reduction of leaf-surface is the 

 absence of markedly dorsi ventral structure. In its extreme 

 form this results in the leaf acquiring radial organisation, 

 i.e. it becomes centric, and it may then closely resemble a 

 stem. Such is the leaf of the Jointed Rush (Juncus articulatus) , 

 whose sheathing base passes over into an almost cylindrical 

 lamina with a slight concavity on the upper side which 

 faces the stem. The transverse section (Fig. 86) presents 

 an epidermis with thick outer walls and a pronounced cuticle. 

 The sunken stomata (St.) are distributed at intervals 



EJ.S. 



FIG. 86. Transverse section of the leaf of the Jointed 

 Rush (Juncus articulatus}. The upper figure shows 

 a diagram of about half the section, the lower a 

 small segment on a larger scale. C., assimilatory 

 tissue ; p., parenchyma ; p.c., central cavity ; St., 

 stoma ; V.b., vascular bundles. 



