268 TRANSPIRATION CURRENT [CH. 



water pores, in other aquatics 1 . Possibly useless or poisonous 

 substances, carried by the ascending sap, which, in the case 

 of plants that get rid of their superfluous water through 

 innumerable stomates, are too much diffused to do damage, 

 may accumulate to a deleterious degree when they are localised 

 by the elimination of the water through a relatively small 

 number of pores. But, whatever its cause, the loss of the water 

 pores of Callitriche seems more than compensated by the result- 

 ing development of "apical openings" (Fig. 163 5). The 



A 



FIG. 163. Callitriche autumnalis, L. A, epidermis of apex of young leaf seen Irom 

 below with a group of stomates. B, apex of an older leaf seen from below. The 

 large opening in the epidermis is due to the resorption of five stomates ; below the 

 opening the small-celled parenchyma is exposed. [Borodin, J. (1870).] 



destruction of the two groups of stomates exposes the sub- 

 stomatal chambers, which communicate directly with the apex 

 of the vascular bundle, and apparently, through these two 

 cavities, water is directly extruded. 



In other cases the apical openings are said to have no con- 

 nexion with the destruction of water stomates. The entire tissue 

 clothing a bundle-end, including the epidermis, disappears, 

 leaving the tracheids actually emerging at the surface. In 

 Heteranthera zosterae/olla, water pores and an apical opening 

 exist side by side, while Fig. 108, p. 167, represents a longi- 

 tudinal section of the leaf-tip of Potamogeton densus^ in which, by 

 the death of the apical cells, the median nerve is brought into 

 direct contact with the water. 



That the elimination of water does actually take place 

 1 Minden, M. von (1899). 



