INFLUENCE OF ENVJKONMENT 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 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 

 sewage or with the products of putrefaction. They are 

 chiefly certain species of Algae 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 



