192 



FRESH-WATER BIOLOGY 



the Nymphaeaceae large tubers are common and young plants of 

 Nymphaea alba may sometimes be found floating about attached 

 to a tuber. 



The swamp plants, such as Typha and Scirpus, also have exten- 

 sive rhizome systems which are important means of wintering and 

 acquiring new territory. 



Some plants have winter buds or hibernacula which form in 

 autumn, separate from the parent plant, often drift to a new 

 locahty, and finally sink to pass the winter 

 in a dormant condition only to commence a 

 new generation the following spring (Fig. 269). 

 Such winter buds are commonly formed by 

 Utricularia, Potamogeton crispus, P zosterifo- 

 lius, P. pusillus, P.frasii, and possibly others. 

 The sinking of those winter buds may be ac- 

 complished by the intercellular spaces becom- 

 ing injected with water, as is the case with 

 the autumn plants of Lemna minor. 



Aside from special organs of propagation 

 quite a few plants acquire new stations by 

 means of the fragments of vegetative parts accidentally set adrift. 

 It is common to find floating stems of Elodea, from the nodes 

 of which adventitious roots have risen. These roots grow straight 

 downward and the stem makes Httle growth in length while the 

 roots are seeking the soil. A fragment of Elodea was found floating 

 in Lake Erie which had an adventitious unbranched root 45 cms. 

 in length. The roots do not branch in some species until the soil 

 is penetrated and then a system of lateral branches develops to 

 anchor the plant. 



In Potamogeton perfoliatus the adventitious roots usually arise 

 from the nodes of new rhizomes which develop in the leaf axils of 

 the cutting. 



With land plants the development of roots on the seedling is as 

 marked as the growth of stem and leaves, but in several water 

 plants the root development is subordinated to that of the stem 

 and leaves, while in some species a genuine functional root is not 

 developed. The rudiment of a root may be present as a part of 



Fig. 269. Potamogeton crispus. 

 Winter bud germinating in 

 the spring. A rhizome with 

 roots and two new shoots has 

 developed. (After Trevira- 

 nus.) 



