THE LARGER AQUATIC VEGETATION 191 



is because they are little exposed and do not need protection against 

 a rapid loss of water. 



Very few species develop a showy corolla under water, but Heter- 

 anthera graminea is one which has a fairly conspicuous flower under 

 water. 



Most of the attached flowering plants are perennial, and vegeta- 

 tive propagation is very common. Naias flexilis is an annual. 



There is a period of rest for water plants just as for land plants 

 and as in the latter so in the former this period occurs during the 

 cold season. Not all of our perennial aquatics make special prep- 

 aration for passing the winter, and some, as Ranunculus aquatilis, 

 Ruppia, and Zannichellia, may be found in normal condition even 

 during the winter. The drifting fragments of Ceratophyllum often 

 become attached by accidental lodgment and pass the winter in 

 the vegetative condition. 



Some Potamogetons, Ranunculus aquatilis, and others will con- 

 tinue to grow uninterruptedly all winter if planted in aquaria and 

 kept at favorable temperature in the greenhouse. 



Vegetative reproduction is the conspicuous method of propa- 

 gation among the larger aquatics, and although many of the species 

 produce seed there are few which could not easily persist if seed 

 production were to be discontinued. In some cases fruit formation 

 has been abandoned. Elodea and Potamogeton robbinsii rarely fruit. 



The rhizomes of most of the water plants are well developed and 

 represent a considerable portion of the vegetation. In some cases, 

 as in Potamogeton perfoliatus, if a plant be taken carefully from the 

 soil fully one-half the fresh weight of the plant will be found to 

 consist of roots and rhizomes. With 

 the approach of cold weather the 

 stems and leaves gradually disin- 

 tegrate but the rhizomes remain 

 alive and pass the winter buried in 

 the mud and in the spring send up FIG 2fig 



Shoots from the bllds previously November with winter buds. (After Irmisch.) 



formed (Fig. 268). Heteranthera graminea has long black rhizomes 

 that are cord-like and often quite tough. The young plants seem 

 in some cases to rise from the runners adventitiously. Among 



