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174 SUPPOSED HYBEIDITY IN POTAMOGETON.
as varying depth of water, or a greater or less degree of tempera-
ture, or whether they grow in stations crowded with other plants.
This race of P. Zizii, which grows in Westmoor, in the
parishes of Chatteris and Doddington, extends over four or five
square miles of the Fen, so that the constantly varying conditions
of the numerous mill-drains and smaller ditches it inhabits afford
unusually favourable opportunities for its study. Some ditch is constantly
growing up, or some drain is deepened, or new ditches and
drains are sometimes made, so that the deep-water lucens-Y\kQ form
of one year, or of early spring, may become the shallow-water
heterophyllus -like form of another year, or of late summer in the
same year even. Now on this west side of the Island of Chatteris
no Potamoijeton heterophyllus is at present to be found. P. Zizii has
lucens, nutans and perfoliatus as its usual companions.
Such forms as P. varians and P. coriaceus, both probable hybrids
between Zizii and heterophyllus, therefore ought not to occur in
Westmoor, and do not. But, on the other hand, a single plant of
P. decipiens [lucens x perfoUatus), apparently a seedling, has been
found, and also another decipiens-\\\e form, in a single plant, which
suggests Zizii x perfoliatus. One other probable hybrid has also
occurred : P. fluitans, or lucens x natans.
This latter form occurred in two distant places, in single plants
in each locality only, as if only recently introduced, or sprung from
seeds.
Now if we turn to the east of Chatteris, we come to a district
called Block Fen and Witcham Headlands. In both these locali-
ties P. Zizii and P. heterophyllus are found constantly growing side
by side, over an area of about the same extent as the Westmoor P.
Zizii occupies. Here we find very diflerent results ; both P. varians
and P. coriaceus occur plentifully, and P. Zizii and P. heterophyllus
both produce numerous permanent forms, some of which are
almost undoubtedly hybrids. In one drain between Block Fen and
Witcham Headlands ("Broker's Drain" of my labels), almost
every group of P. Zizii and P. heterophyllus seems to be distinct
from its neighbouring group ; some forms appear to be exactly intermediate,
others approach more nearly to one or other of the
supposed parents ; but few of them seem to be states, as I have not
found them to revert to either of the type species, but they remain
constantly distinct from one another through the most varying conditions.
Some of these forms are no doubt true varieties, but
others are quite as certainly hybrids, or double hybrids between
Zizii and lieteropliyllus.
Although for a year or two past I had begun to suspect a
remarkable form growing in Broker's Drain to be of hybrid origin,
it was not until the summer of 1889 that I succeeded in obtaining
what seems to be a direct proof of the hybrid parentage of any one
individual.
For three or four years I had watched the growth of a slender
plant, apparently a seedling, also growing m Broker's Drain, and
had placed it in its immature state as a form allied to P. heterophyllus
var. jiuctuans of Tiselius ; the plant had not flowered, nor