440 VENTILATING SYSTEM 



water-vapour has to traverse in escaping from the interior of the leaf 

 to the outer atmosphere ; instead of pursuing a straight or at most 

 a slightly curved course, the vapour has to make its way through 

 successive layers of palisade-tissue by a long and tortuous path. 



4. The ventilating arrangements of marsh- and water-plants. 



In aquatic plants which are completely and permanently submerged, 

 the entire gaseous interchange goes on without the aid of stomata, by 

 diffusion through the whole of the outer surface, just in the same way 

 as the respiratory gas-exchange of gill-breathing aquatic animals. 



The disadvantage of this cuticular method of gas-exchange consists 

 in its extreme slowness. There are two prominent features in the 

 construction of submerged plants which tend to counteract this 

 drawback. Diffusion will obviously be accelerated if the superficial 

 area of the plant- body is increased. As a matter of fact, the external 

 surface is very often amplified in water-plants in a variety of ways. 

 Most frequently the leaves are subdivided into a number of narrow 

 segments {Myriophyllum). In Ouvirandra fenestralis the leaf-blade is 

 perforated so as to form a lattice- work, while in other cases again 

 (e.g. Elodea canadensis) the leaves remain undivided, but become 

 correspondingly thin and delicate. According to Goebel, the seedlings 

 of Euryale and Victoria are provided with localised surface-expansions, 

 which serve to accelerate gaseous interchange ; here the resemblance 

 to the gills of animals is very pronounced. 214 In the case of 

 Euryale, the abortive primary root bears a four-lobed gill-organ, 

 which persists until the seedling has fully expanded and has come 

 into contact with water on every side ; each of the lobes is branched 

 and thickly covered with hairs. Goebel believes that the organ is 

 concerned with the absorption of oxygen during the early stages of 

 germination ; although this view has not been put to the test of 

 experiment, there is much to be said in its favour. 



The inconvenience consequent upon the slowness of gaseous 

 interchange under aquatic conditions, may also be mitigated by the 

 development, in submerged organs, of large internal air-chambers. The 

 living tissues can thus maintain an active photosynthetic or respiratory 

 interchange with the " internal atmosphere " provided in this way. It 

 must be noted, that the entrance of carbon dioxide into the plant from 

 the surrounding water takes place more rapidly than the out ward 

 diffusion of oxygen ; the photosynthetic gas-exchange will therefore be 

 accelerated, if extensive internal air-spaces are provided for the 

 temporary accommodation of comparatively large quantities of oxygen. 

 If shoots of Myriophyllum , Geratophyllum and similar plants are cut 

 across while they are engaged in photosynthesis, streams of bubbles, 



