xxi] APICAL OPENINGS 269 



through the apical openings of submerged leaves, is indicated 

 by certain observations made independently by two different 

 workers at the end of the last century^. In the natural situation 

 of the leaves, it is not easy to devise a means of rendering this 

 elimination visible, but it is found that, if the level of the water 

 surface be lowered until the leaf apices emerge into the air, 

 drops of water appear in the region of the apical opening; if 

 wiped away they speedily re-form. This phenomenon has been 

 witnessed in a considerable number of cases as, for instance, 

 the submerged leaves of Littorella and Potamogeton crispus and 

 we shall probably not be guilty of too great an assumption in 

 supposing that the same thing goes on when the leaves are 

 beneath the water surface. The exudation of water from water 

 pores has been shown, in the case of land plants, to be dependent 

 upon root-pressure, and the existence of identical pores in 

 submerged species lends colour to the view that the roots of such 

 plants are not mere holdfasts, but have to some extent retained 

 their function as organs of absorption. 



Notwithstanding the advances that have been made, many 

 problems connected with the absorption and elimination of 

 water by submerged plants remain to be solved. In Hydro cleis 

 nymphoides^ for instance, by the disappearance of a special 

 transitory tissue at the leaf apex, the tracheids are left communi- 

 cating freely with an empty space, but this space remains separated 

 from the water by a persistent roof of cuticle, and can there- 

 fore play no part in the elimination of water (Fig. 1 64, p. 2 70)^. 

 Again, side by side with Zostera^ whose leaves are provided with 

 apical openings, we have two other marine genera of the Pota- 

 mogetonaceae, Cymodocea and Posidonia^ in which no such open- 

 ings occur. It seems that we must either suppose that the 

 elimination of water from the apical openings is of relatively 

 little importance, or that, in related genera, the main physio- 

 logical activities of the plant may be differently performed. 



In the case of such submerged, rootless plants as Cerato- 



1 Minden, M. von (1899) and Weinrowsky, P. (1899). 



2 Sauvageau, C. (1893). 



