INFLUENCE OF ENVIRONMENT ON PLANTS 335 
which the protoplasts require. These intercellular reser- 
voirs 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 morganic 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 
sewage or with the products of putrefaction. They are 
chiefly certain species of Alge or Fungi, but among them 
may be included a few Mosses and Phanerogams. 
Another class of plants which show a definite response 
in their structure to the conditions in which they live is 
