OF NAIAS GRAMINEAj VAR. DELILEI. 37 



their lower portion (figs. 64 & 65), thickened in the region 

 of the midrib (figs. 60-63), ^^^l slightly keeled on their 

 lower surface ; in length they vary from f inch to 1 1 inch, 

 and they are — • inch broad or less (see Plate IV. fig. 2) . 

 The sides of the fully-developed leaf are parallel for the 

 greater portion of their length, but at their base they 

 widen out into a broad sheath bearing two upright auricles 

 applied to the stem and half-clasping it (figs. 52-55). The 

 extremity of the leaf is gradually attenuated, and ends in 

 from one to three spines (fig- 43) ; the extremities are 

 frequently truncate, so that the spines give it a cuspidate 

 character (fig. 44). 



The margins of the sides, sheath, and free extremity are 

 studded with erect, unicellular, yellowish-brown spines 

 (figs. 47-49), whose colour presents a contrast to the 

 transparent marginal cell-walls, and to the green contents 

 of the cells of the lamina of the leaf. The spines are 

 acuminate, slightly curved, and gradually narrowed from 

 the base to the sharp point. 



VI. The Leaf-spines. 



The form of the spine, or tooth, on the margin of the 

 leaf furnishes good discriminating characters between the 

 various species of Naias, as was long ago pointed out by 

 the late Al. Braun in, the ^Journal of Botany,' vol. ii. 1864, 

 pp. 274-279. 



The simplest form of tooth is that of N. flexilis, where, 

 in Dr. BoswelVs Loch-Cluny specimens, the base of the 

 spine is in the same plane as the leaf-margin. The spine 

 springs from a dilatation between two of the marginal 

 leaf-cells (fig. 45), each of which nearly equally supports 

 the spine to the extent of one third its length, rarely 

 more. Sometimes the two marginal cells are separated 

 from each other by the spine (see fig. 46). 



