336 COMMON WEEDS 



Arrow-head (Sagittaria sagiltifolia L.) is another 

 species of this order less common than Water Plantain, 

 but met with in similar positions in water. It has 

 peculiar creeping stoloniferous stems, the branches of 

 which end in small tubers about half an inch in 

 diameter. The sub-aerial leaves are characteristically 

 arrow-shaped, hence the popular name of the plant. The 

 leaves in the water consist of narrow petioles without 

 leaf-blades. 



The unisexual flowers have white petals, and are 

 arranged in whorls ; the upper are male, and the 

 lower female. 



NAIADACE^: 



Pondweed. The term "pondweed" is applied in a 

 restricted sense to representatives of the genus Potamo- 

 geton. A large number of species are known, some of 

 them with floating leaves, others with submerged 

 leaves only. The majority are inhabitants of water 

 from 12 to 24 feet deep. 



The flowers are arranged in terminal or axillary 

 spikes ; they are small, with four greenish perianth 

 segments, four stamens, and an ovary of four carpels, 

 which ripen into small drupes, each containing a single 

 seed. 



The species perhaps most frequently met with 

 are : 



Potamogeton natans L. (Fig. 98), with floating, leathery, 

 elliptic or ovate leaf-blades and long alternate petioles, 

 some of which remain under water and develop no 

 blade. 



P, polygonifolius Pourr. somewhat resembles P. natans, 

 but has narrower floating leaf-blades, and lanceolate 

 submerged blades also. 



P. heterophyllus Schreb. has flowering stems with 



