AQUATIC PLANTS OF FRESHWATER 



the epidermal cells of the foliage leaves and not by roots, though the lower 

 leaves may assume a rootlike character. The Hornworts have weak and 

 slender widely-branching and floating stems with verticulate leaves, which 

 appear in thick clusters on the younger growth and branches. It is not a 

 desirable aquarium plant as it is too fragile and requires careful attention, 

 may decay in a brief period, contaminate the water, and cause the death 

 of the fishes. Several species have been proposed, based on the spurs, 

 spines or wings of the fruit but none of these distinctions are of value. 



Ceratophyllum demersum (Gray), or Common Hornwort, Fig. 125, 

 has long or short slender submerged floating stems, dependent upon the 

 depth of the water, as it is characteristic of the plant to form thick mats 

 immediately below the surface. The linear 

 2 to 3 times divided and forked leaves grow 

 in verticils of 5's to 12's on a weak and 

 fragile stem. The flowers are insignificant, 

 either white or yellowish in color, of which the 

 male and female are distinct. The former 

 consists of about 15 sessile anthers and the 

 latter of a small one-seeded ovary, but both 

 surrounded by a whorl of very small bracts, 

 while the fruit is oval with either a straight 

 and spinelike beak, smooth or with a basal 

 spur, or tubercular with narrow winglike spiny 

 lower margin. Grows freely in ponds and 

 slow streams throughout North America 

 except in the extreme North. European 

 authorities mention two other species, C. 

 submersum and C. platyacanthum, not gener- 

 ally known in the United States. They are 

 probably natural variants of C. demur sum the 

 universally distributed species. 



Podostemon ceratophyllum (Linn.) is 

 another aquatic bearing the name Cerato- 

 phyllum. It is popularly known as River 

 Weed or Threadfoot, and is a dark green 

 rather stiff plant, firmly attached to the 

 stones in running streams. The densely 

 tufted leaves are narrowly linear and sheathed at the base, but split above 

 into filiform segments. It resembles C. demersum but is coarser and 

 rougher in appearance. The white flowers are ^ to ^ inch broad, 

 spreading from the spathes; and the oblong-oval capsules are borne on a 



FIG. 125. Hornwort, Ceratophyllum 



demersum, whorled leaf, blossoms and 



enlarged seed carpel. Reduced one-third. 



198 



