374 ECOLOGY 



and therefore not readily eaten by such aquatic animals 

 as snails. The epidermis is usually very thin, and enables 

 the plants to absorb through it much of their food in the 

 form of mineral salts and carbon dioxide dissolved in the 

 water in which they live. 



Invasion. Water-plants as land-winners. Plants growing 

 in water are not subjected to such extremes as plants 

 growing in the air, and consequently grow rapidly ; the 

 rhizomes plough their way through the mud and give off 

 innumerable shoots into the water. This rapid vegetative 

 growth enables the plants to spread quickly, and they are 

 further aided by the ease with which detached shoots grow 

 and form new plants. On the approach of winter, special 

 winter buds are formed in many species, such as Pond-weeds 

 and Frog-bit ; these break off, fall to the bottom, and rest 

 until the following spring. By such vegetative means, 

 rather than by seeds (see p. 142), water-plants reproduce 

 themselves extensively, and rapidly invade the water. 



Interesting examples of this may be found in many 

 ditches, ponds, and lakes. Reed-like plants may be seen 

 to extend from the margin across the inlet. Their stems 

 and leaves form a filter, keeping back the mud, which, 

 together with the remains of successive seasons of plants, 

 chokes up the channel and provides a soil over which the 

 plants from the banks push their way farther and farther. 

 Thus in time the pond or lake becomes converted into 

 a marsh. 



By such invasion, plants become important land-winners. 

 The aquatic vegetation by its very luxuriance prepares the 

 way for its own extinction. It contributes to the changes 

 of conditions which favour a drier type of plant -life, and 

 it is only a matter of time when it will be succeeded by the 

 invaders. If further changes should result in improved 

 drainage, the marsh type will itself be supplanted. In 

 this way invasion and succession follow each other in 



