l8o FRESH-WATER BIOLOGY 



been known for a long time and a great deal of attention has been 

 given to a study of its structure. The vascular system of aquatics 

 is much simpler than that of land plants and seems to represent a 

 degenerate type of the latter. This general fact has thus far been 

 interpreted uniformly as indicating that a conductive tissue is 

 useless in water plants. By logical inference such plants were once 

 terrestrial but degeneration of the vascular system has accom- 

 panied adaptation to the aquatic habit. A very different interpre- 

 tation may, however, easily be made. The significant fact is, 

 that even those plants which live wholly submerged and are with- 

 out organs of attachment show at least the rudiments of a con- 

 ducting system. But why should such plants have any vascular 

 tissue at all? The epidermis is thin and permeable to solutions of 

 mineral matter, the tissues are usually only a few cells in thickness, 

 and in plants without roots, as Ceratophyllum, absorption must take 

 place in such a large number of the cells that a special tissue system 

 for the conduction of water is unnecessary. 



An aquatic environment does not favor the great differentiation 

 of tissue characteristic of terrestrial plants. When in water plants 

 very simple imitations of the land plant structure are found, this 

 condition does not represent the extreme that has been developed 

 through a long succession of aquatic ancestors, but is to be re- 

 garded as- indicating the tendency toward simplification made 

 necessary by increasing adaptation to the water life. From this 

 point of view the conductive tissue is becoming, rather than has 

 become, unnecessary. So it seems probable from anatomical study 

 that a simplification of the vascular system is in progress which, if 

 continued, will eventually lead to a suppression or total disappear- 

 ance of special conductive tissue. At present, however, it may 

 safely be said that the majority of our larger water plants have 

 need of vascular tissue. 



The leaves of water plants may be either floating or submerged. 

 Some plants have only the floating or only the submerged, while 

 several species have both kinds on the same plant at the same time. 

 The floating leaves do not show a great variety of form and tend to 

 be elliptical, oval, or round, while some are shield-shaped. Since 

 an aquatic environment is more uniform one cannot expect as 



