542 
POTAMOGETON 
ning with the floating sp. and ending with the narrow-leafed sub* 
merged ones (see p. 160). There can be no doubt of the origin of 
the P. from land plants, and Schenck looks upon P. natans L. as the 
sp. least modified to suit a water existence, i.e. the nearest to the 
ancestral type. The upper leaves are ovate, leathery, and float on the 
water; the lower are submerged, and sometimes reduced to a linear 
form. Then come such sp. as P. heterophyllus Schreb. where the 
submerged leaves are all narrow. Next P. lucens L., P. crispus L., 
&c. with all the leaves lanceolate and submerged. Then in P. ob - 
tusifolius Mert. et Koch, P. pusillus L., &c., the leaves are narrow 
and of a long ribbon shape. P. trichoides Cham, et Schlecht. repre- 
sents the most highly modified type of all. [All the above are Brit, 
sp. ; they are best studied at first in a herbarium, for P. is as variable 
a genus as Rubus or Hieracium, and the sp. are exceedingly difficult 
to determine. The fact is that probably the genus is still in a condition 
of rapid evolution of which the sp. above mentioned represent various 
stages.] Interesting phenomena connected with the above are to be 
seen in the germination of the seeds. The internal anatomy also 
shows interesting features in the series of types mentioned. 
Hibernation occurs in different ways; some sp. remain green all 
winter ; P. natans , &c. die down and leave only the rhizome ; P. pec - 
tinatus L. forms peculiar tubers on special branches (Schenk, Wasser - 
gewachse , p. 86) ; P. crispus and others form winter buds with broad 
leaves (not closely packed as in Utricularia &c., but wavy, like holly 
leaves) ; P. obtusifolius forms winter buds of the ordinary kind. The 
firs, are of simple structure, arranged in spikes which project above 
the water. Each has 4 sta. in two whorls, and 4 cpls. From the 
connective of each sessile anther there grows out a cup-shaped ex- 
pansion simulating a perianth leaf. The fir. is protogynous and wind- 
fertilised. The outer layer of the pericarp contains air, so that the 
fruit, which is an achene, floats on the water and may thus be carried 
to a distance, finally sinking when the air escapes. [For further details 
see p. 158 and Literature.] 
Potamogetonaceae. Monocotyledons (Helobieae). 9 gen. with 70 sp. 
cosmop. All are water plants, several of them being marine. There 
is in most a creeping stem or rhizome, mono- or sym-podial, attached 
to the soil by adventitious roots, and sending off erect branches up- 
wards into the water. These usually have ribbon leaves, submerged 
(exceptions occur in Potamogeton), arranged in \ phyllotaxy. The 
base is sheathing, and within the sheath are the small scales ( squamulae 
intravaginales ) which occur in most of the orders of Helobieae. The 
infl. is a spike or cyme, or the firs, solitary. The firs, possess no true 
perianth (exc. Zannichellia ? ), but Potamogeton shows leaf-like out- 
growths from the sta. which perform the perianth functions ; similar 
structures occur in Zostera and Ruppia. In several genera it is not 
easy to decide what is to be regarded as the fir. (see Zostera). 
