HIGHER PLANTS 



Wolffia Columbiana.— The Wolffias (Fig. 70) are the small- 

 est of all the flowering plants. They float just below the 

 surface film, minute green grains often wedged in among 



Fig. 70. — Wolffia columhiana. (From Britton 

 and Brown.) 



the duckweed plants. The thallus is about a millimeter 

 long, and Wolffias are easily recognized by this minute size 

 and the entire absence of roots. In autumn they become 

 so laden with their food store of starch grains that they sink 

 to the bottom and winter over there, rising in the spring after 

 they have used up their own load. 



Occurrence. — In quiet water, often among duckweeds. 

 June-July. Occasional. Distributed over the United States, 

 less frequent near the Atlantic coast. 



Pickerel-weeds — Pontederiacece 



Pickerel-weed, Pontederia cordata. — Pickerel-weeds are 

 persistent invaders of water territory. Wherever brook-fed 

 ponds are being filled up they are usually in the front ranks of 

 vegetation which is crowding into the water (PI. V). They 

 grow I to 3 feet high with thick, broadly arrow-shaped leaves. 

 Their violet-blue flowers are crowded together in spike-like 

 clusters about 3 inches long; these fade so quickly that com- 

 paratively few perfect spikes of flowers can ever be found 



(Fig. 71). 



85 



