1898.] 



on a Yorlishire Moor, 



633 



moorland grasses (Nardus, Aira flexuosa, Festuca ovina) are per- 

 manently in-rolled, and flatten out very slowly and imperfectly, even 

 when immersed in water for many hours. 



Onr moorland grasses are all arctic, and occur both in the old 

 and the new worlds ; Festuca ovina is also a grass of the steppes ; it 

 is world-wide, being found in all continents, especially on mountains, 

 and even reaching Australia and New Zealand. 



It may seem paradoxical to count the Rushes as plants which are 

 protected against drought, for they often grow in the wettest part of 

 the moor. They are common, however, in dry and stony places, and 



Fig. 10. 



-Transverse section of leaf of Nardns stricta^ 

 showing permanent in-rolling. 



their structure is completely xerophytic. The leaves are often 

 reduced to small sheaths, which wither early, while the stems are 

 green, and perform the work of assimilation; or else, as happens in 

 certain species, the leaves assume the ordinary structure of the stem. 

 The cylindrical form of the rush stem is significant, for of all 

 elongate solid figures the cylinder exposes the smallest surface in 

 proportion to its volume. Moreover a cylindrical stem, without 

 offstanding leaves, and alike on all sides, is w^ell suited, as Jungner 

 points out, to the circumpolar light, which shines at low angles from 

 every quarter in succession. A rush stem is singularly dry, the 



