Readaptations to Life in the Water 271 



4. In general: the following characteristics: 



a. The production of abundance of mucilage, 

 which, forming a coating over the surface, may 

 be of use to the plants in various ways : 



1. For flotation, when the mucilage is of low 

 specific gravity. 



2. For defense against animals to which the 

 mucilage is inedible or repugnant. 



3. For lubrication: a very important need; 

 for, when crossed plant stems are tossed by 

 waves, the mucilage reduces their mutual 

 friction and prevents breaking. 



4. For preventing evaporation on chance ex- 

 posure to the air. 



5. For regulating osmotic pressure, and aiding 

 in the physical processes of metabolism. 



b . Development of vegetative reproductive bodies : 



1. Hibernaculea, such as those of the bladder- 

 wort (fig. 162). 



2. Tubers such as those of the sago pondweed 

 (see fig. 228), the arrow-head, etc. 



3. Burs, such as terminate the leafy shoots of 

 the ruffled pondweed (see fig. 63). 



4. Offsets and runners, such as are common 

 among land plants. 



5. Detachable branches and stem segments, 

 that freely produce adventitious roots and 

 establish new plants. 



c. Diminished seed production. This is correlated 



with the preceding. Some aquatics such as 

 duckweeds and hornworts are rarely known to 

 produce seeds; others ripen seeds, but rarely 

 develop plants from them. Their increase is 

 by means of the vegetative propagative struct- 

 ures above mentioned, and they hold their 

 place in the world by continuous occupation 

 of it. 



