THE LARGER AQUATIC VEGETATION 



185 



submerged 



of the plants bearing them were adapted to life on land or 

 at least lived under exposure to loss of water by transpiration. 



Other openings in the leaf have also been found in 

 some species. These occur at the apex of the leaf 

 more frequently in the submerged leaves than in 

 the floating ones. The opening does not show any 

 special structure, as is true of stomata, and is formed 

 by the decay and falling away of the tissue at the 

 apex, so that the conductive vessels in the veins of 

 the leaf become exposed to, and in direct communi- 

 cation with, the water. In some cases this disin- FlG - 26 - 



Tcpcfts. 



tegration of tissue at the apex may go so far as to 



change to a marked degree the shape of the apex, (After Sauva * eau -) 



making it rounded instead of pointed (Figs. 261 and 262). 



The formation of the opening seems to occur before the leaf 



matures but is seldom found on the young leaves. In addition to the 



species already known as bearing these 

 openings the writer may mention that 

 of V ' allisneria spiralis on whose half- 

 grown leaves he has observed them. 

 Some authors have suggested that the 

 passage of water through the conduc- 

 tive tissue is facilitated and that the 

 excretion is aided. This is really a 

 supposition and has never been proved. 

 The presence of an earthy coating on the leaves and stems of 



some water plants may be commonly ob- 



served. This mineral incrustation appears 



like a coating of mud on the leaf in many 



cases, while in others it is not so conspicuous 



and is only noticeable when the. plant is 



handled. Only the submerged organs seem 



, it . FIG. 262. Potamogetondensus. Leai in 



tO bear the incrustation, even the lower longitudinal section. The decayed 



. tissue has fallen away, leaving the 



Surface Of floating leaves being leSS faVOr- vessels exposed to the surrounding 



water. X about 135- (After 



FIG. 261. Zostera nana. Apical portion of 

 a mature submerged leaf, showing the 

 change of form at the apex due to decay 

 of apical tissue. X about 40. (After 

 Sauvageau.) 



able to its formation and much less fre- 



quently bearing it. Potamogeton pectinatus is seldom, if ever, 



incrus ted, 'while other species of this genus usually are. Char a is 



