CENTRIC LEAVES 



165 



tective hairs can be gauged by comparing the rates of evaporation 

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

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

 about I J 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 feature 



which often accompanies a reduction of leaf-surface is the 

 absence of markedly dorsiventral 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 [J uncus articnlatus) , 

 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 



Fig. 



E-J.S. 



86. — Transverse section ol the leaf of the Jointed 

 Rush (Jiincus articnlatus). 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. 



