342 VEGETABLE PHYSIOLOGY 
Jacune, 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 organic 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 
