STEMS 



677 



in which considerable surplus food accumulates; the rhizomes of bul- 

 rushes, cattails, and flags, though much smaller, are large as compared 

 with most rhizomes, yet they migrate with some rapidity. 



The remarkable capacity of water plants for vegetative reproduction 

 is due chiefly to the ready detachability of aquatic stems, whose frag- 

 ments float to a more or less distant locality, where a new growth center 

 is established. A striking case of vegetative reproduction among hydro- 

 phytes is seen in Eichhornia (the water hyacinth), which in recent years 

 has filled various streams in Florida to such an extent as to impede navi- 

 gation; another remarkable example is afforded by Elodea canadensis 



Figs. 994-996. — Multicipital stems in the dandelion {Taraxacum officinale): 994, a 

 young plant, showing a simple tap root (r) crowned with a single rosette of leaves ; 995, 

 an older plant, showing a larger root, crowned with a multicipital stem (s) with three leaf 

 rosettes ; 996, a much older plant, showing how a dandelion plant eventually may break 

 up into several individuals. 



(often called the water pest, because of its rapid filling of ponds), which, 

 from a single plant introduced from America, spread over Europe 

 within half a century ; this rapid spread probably was due entirely to 

 vegetative reproduction, since only pistillate plants of Elodea are known 

 in Europe. In Elodea, spreading is due to the fragmentation of ordi- 

 nary shoots, but in Eichhornia there are aquatic runners which give rise 

 to readily detachable new plants. Another rapid spreader is the water 

 cress, which soon fills a spring brook when planted at the head. In the 

 duckweeds new thalli develop from the old (figs. 997-, 727), becoming de- 

 tached with such rapidity that the vegetative offspring of a few plants 

 may fill a small pond in a short time. In rivers, migration is, of 

 course, much more rapid downstream than up, yet water plants may 



