342 VEGETABLE PHYSIOLOGY 



lacunae, with which the smaller spaces communicate, form a 

 mechanism by which this difficulty is surmounted, affording 

 reservoirs of air of considerable size in the interior of all 

 parts that are submerged, so that the slow rate of renewal of 

 air does not impede the gaseous interchanges which the 

 protoplasts require. These intercellular reservoirs are not 

 confined to the vertical stems, petioles, and leaves, but 

 occur also in the more woody stems or rhizomes which 

 "many of these plants possess (fig. 142). 



The absence of the transpiration current appears to be 

 correlated with a comparatively small development of the 

 plant-body. The large quantities of inorganic salts which 

 the dilute solutions absorbed by the roots carry into the 

 plant, in cases where the total absorption is very great 

 owing to a large transpiration, lead to a large increase of 

 constructive activity. In the absence of such an enormous 

 absorption the plant-body does not receive the materials 

 necessary for the acquirement of a considerable bulk. 

 Aquatic vascular plants are consequently never very large. 



The difference between the two groups of aquatic plants 

 spoken of may be well seen in such forms as Cabomba, 

 which bears both submerged and floating leaves. These 

 show respectively the characteristics described in each case. 



Some curious adaptations of the organism to its environ- 

 ments are exhibited by certain of these plants which live 

 in marshy surroundings, sometimes being nearly or wholly 

 submerged, and at others, owing to the drying up of the 

 water, growing upon the mud. When the latter fate befalls 

 them, such of their leaves as are adapted to an aquatic life 

 become dried up and perish. The upper leaves, which have 

 always been exposed to the air, do not suffer. As growth 

 continues, all the foliage which is produced is of the terres- 

 trial type. On the other hand, when the plant-body is 

 submerged the new leaves are all of the aquatic type. 

 These plants are often spoken of as amphibious. 



Some aquatic plants are saprophytic in their mode of 

 life, flourishing best in water which is contaminated with 



