FIELD AND FOREST, 23 



the Polygala Curtisii, a straggling mikwort not yet collected elsewhere. 

 The specimens of Curtis, on which the species was established, appear 

 as if somewhat drawn by growing in the shade, subsequent specimens 

 referred to it being more compact, but possessing quite distinct char- 

 acters in the seed. Chondrilla. juticea is abundant in the District of 

 Columbia, and its branching stems, entirely destitute of leaves when 

 its composite flowers appear, look very singular when first seen. Like 

 chicory, around Boston, it promises to become one of our most com- 

 mon road-side weeds. It is usually accompanied by Calamintha m- 

 peta, a slender plant of the mint family. In a field near Mount Pleas- 

 ant, on Rock Creek grows Orobauche minor, supposed to be a parasite 

 on the roots of clover. It is introduced, and perhaps a temporary vis- 

 itor, but hardly found anywhere else in this country A few orchida- 

 ceous plants are quite common, as Aplectrmn, Tipularia and Aphyllon, 

 but as a whole, the family does not flourish here. And finally Obola- 

 ria can be found plentiful on the hills northwest of Washington; a lit- 

 tle succulent leafy herb that forms rather a pretty ornament in a fern- 

 ery, continuing to flower for several weeks. 



Few rivers in the older parts of our country are so little disturbed 

 by man as the Potomac. A Congressional commission authorized to 

 find a water supply for the city of Washington, reported in 1853 that 

 the Potomac at Great Falls, 16 miles above the city, was the most 

 available source. The report was adopted, and in 1855 one of the 

 finest aqueducts in the world was begun, the bridge by which it cros- 

 ses Cabin John creek being the longest span of stone ever built. 



Whatever may be in the future, the purity of the Potomac water 

 above the Great Falls is as yet only affected by natural causes, No 

 manufactories line its banks, and no large towns pour in their streams 

 of filth to poison the water below them ; but in early Summer, as last 

 June, it sometimes acquires a fishy taste and smell, very unpleasant, if 

 not positively unhealthy. Every body suddenly becomes interested 

 in the natural history of the Potomac, After a few days the water 

 usually returns to its normal condition, and the cause is therefore, 

 temporary, and yet of considerable importance, to affect so large a body 

 of water. The debris from trees along the banks, and general drift 

 stuff" in the river must be pretty uniform in character. 



The plants which grow in the water may be roughly divided into 

 two classes, amphibious and aquatic. The first frequent the muddy 

 flats, or ground only at times submerged, the second are always cov- 



