THE LARGER AQUATIC VEGETATION 



i8S 



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- f^o. 260. zanmchdua 



' repens. Submerged 



tegration of tissue at the apex may go so far as to !fj^' '^'x^'ibout'™' 

 change to a marked degree the shape of the apex, '^"" sauvageau.) 

 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 Vallisneria 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 incnistation 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 

 to bear the incrustation, even the lower 

 surface of floating leaves being less favor- 

 able to its formation and much less fre- 

 quently bearing it. Potamogeton pectinatus is seldom, 



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 

 SauvageauJ 



Fio. 262. Polamogelon densus. Leaf in 

 longitudinal section. The decayed 

 tissue has fallen away, leaving the 

 vessels exposed to the surrounding 

 water. X about 135. (After 

 bauvageau.) 



incrusted, while other species of this genus usually are. 



if ever, 

 Chara is 



