174 SUPPOSED HYBRIDITY 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 Westrnoor, 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 con- 

 stantly growing up, or some drain is deepened, or new ditches and 

 drains are sometimes made, so that the deep-water lucens-like 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 Potamogeton heterophyllus is at present to be found. P. Zizii has 

 lacens, natans 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 

 Westrnoor, and do not. But, on the other hand, a single plant of 

 P. decipiens {lucens x perfoliatus), apparently a seedling, has been 

 found, and also another deripiens-hke 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 



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 Westrnoor P. 

 Zizii occupies. Here we find very different 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 Meadlands ("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 in- 

 termediate, 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 con- 

 ditions. Some of these forms are no doubt true varieties, but 

 others are quite as certainly hybrids, or double hybrids between 

 Zizii and heterophyllus. 



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 hi its immature state as a form allied to P. hetero- 

 phyllus var. jiuctuans of Tiselius ; the plant had not flowered, nor 



