THE ECOLOGY OF PLANTS. 421 



all transitions from broad floating leaves (e.g. P. natans, the commonest 

 species) to long narrow submerged ones. P. natans is the species 

 least adapted for aquatic life (and therefore the nearest to the ancestral 

 land type from which the Pondweeds arose) ; its upper leaves are 

 oblong, stalked, and floating, while the lower ones are smaller or 

 reduced to a mere stalk. In P. heterophyllus the upper leaves float but 

 are narrower than in P. natans, and the lower ones are submerged, 

 very narrow, and 2-7 inches long. In P. lucens all the leaves are 

 narrow and submerged, or there may be a few floating upper leaves. 

 All the other British species have all the leaves submerged and either 

 oblong (e.g. P. crispus) or ribbon-like (e.g. P. pusillus, P. pectinatus). 

 The flowers, which come above the water, are in a spike, each flower 

 having 4 stamens, each with a sepal-like scale growing from the outer 

 side of the anther. 



Several plants resemble the Common Pond weed (P. natans) in having 

 broad floating 1 leaves, usually entire and rounded in outline. In 

 the Water-lilies the large leaves are kidney-shaped or almost circular 

 and have long stalks springing from a stout rhizome which sends roots 

 into the mud ; the blade is leathery, with unwettable cuticle, stomates, 

 and palisade tissue on the upper side ; air-chambers are present in all 

 parts of the plant, and the large flowers are on a long flexible stalk. 

 Examine the common Yellow Water-lily or Brandy-bottle (NupTiar 

 luteum) and the White Water-lily (Nymphaea alba), or some of the 

 cultivated species ; note the numerous stamens and carpels, and the 

 transitional structures between petals and stamens found in the 

 Nymphaeas. 



Frogf-bit (Hydrocharis), found in ditches and ponds, has slender 

 stems floating in the water and bearing at intervals tufted shoots, 

 each tuft consisting of several stalked leaves with nearly circular 

 blades, white flowers carried above the surface (male and female 

 on separate plants), and usually a few roots hanging into the 

 water ; resting-buds are formed on the stems, drop to the bottom in 

 autumn, and float up in spring to produce new shoots. Duckweed 

 (Lemna), which often completely covers the surface of ponds, has flat 

 green floating shoots, not distinguished into stem and leaf except that 

 the branches and the minute flowers arise from lateral pockets at the 

 narrow end of the "leaf," which bears long roots below, dangling in 

 the water ; in autumn resting buds are formed in the pockets and the 

 whole plant falls to the bottom. [Other free-floating aquatics, not 

 British but often cultivated, are Water-nut (Trapa), Eichhomia, 

 and Salvinia (a fern).] 



Several British aquatics have all their leaves submerged and 

 finely divided into hair-like segments, like the submerged leaves of 

 the Water Crowfoot. Water Milfoil (Myriophyllum) has a creeping 

 fixed rhizome giving off shoots which float in the water and vary in 

 length according to the depth and current, only the inflorescences 

 coming to the surface ; the leaves are in circles of 3-5, each leaf 

 being pinnately divided, i.e. consisting of an axis bearing a row 

 of narrow segments on either side. Water Hornwort (Cerato- 

 phyllum) resembles Water Milfoil in habit, but the leaves are twice 



