NOTES ON PONDWEEDS. 35 



plants tinder the latter species as an aggregate. But I further thmk 

 it would be most in accordance with what has already been done in 

 the lucens-gYovi^ to make a distinct "species" of this multiform 

 plant, as originally proposed by Mr. Morong. 



The suggestion as to the possible hybridity of P. iY<?-zrt??s remains 

 to be noticed. For a long time I was unable to observe any facts 

 which pointed to any of our fenland varieties of Potamogeton being 

 the result of cross-breeding between "species" more or less nearly 

 allied. But observations made during the past summer strongly 

 induce me now to believe that lucens, heterophijUiis, and Zizii do 

 occasionally interbreed, and also that P. lucens crosses with P. jy^;-- 

 foliaUis. If, however, we regard P. varians as heterophyllus and 

 Zizii, it is somewhat remarkable that such exactly similar plants 

 should be produced on both sides of the Atlantic, especially when 

 we call to mind that the ^^ heterojjliyllus'' and ^' Zizii" of the United 

 States are not by any means like those of Cambridgeshire, but have 

 a distinct facies of their own in all the specimens I have seen. 

 Now, on the contrary, P. varians from the two countries is so 

 exactly alike that if the specimens in my herbarium were mixed 

 together, no botanist could separate them ! Yet so susceptible is 

 varians to local influences that, although in America, where it does 

 not fruit, it is very constant to type, in Cambridgeshire examples I 

 can name the ditch from which each specimen was gathered at a 

 glance, even if the localities are separated by a few yards only. 

 Here we have exact resemblance between the two plants which 

 represent the type described by the Eev. T. Morong, combined with 

 extreme variability from seed in the locality where the plant fruits. 

 This variation in the progeny of the fertile plant is often a marked 

 feature in hybrids between closely-allied forms. We often see the 

 same instability in the artificially-crossed vegetables and flowers of 

 our gardens, before the newly-produced form becomes sufficiently 

 " fixed " to come true from seed. I am aware that it may be objected 

 that the cases are not parallel; that the garden-plants are not 

 hybrids between distinct "species." Possibly so: but what is a 

 distinct species in Potamogeton ? No one really knows — those who 

 know the genus best will find themselves the least able to answer 

 the question with any great degree of confidence. I know of only 

 some half-dozen British forms of the genus so distinct that one does 

 not feel doubtful as to the exact limits we may safely assign to them. 

 P. lanceolatiis Smith is one of the most isolated forms w^e have — so 

 much so that I know not where to place it in the genus. Yet that 

 acute botanist, Mr. W. H. Beeby, suggests to me a possible hybrid 

 origin for this distinct-looking plant, that I cannot gainsay, but 

 which, if proved to be correct, would raise the question of how far 

 cross-breeding has helped to make the genus what we now see it. I 

 repeat : let us name all definable forms the origin of which we cannot 

 reasonably trace ; this will lead to their examination and study, and 

 possibly to direct experiment in crossing certain species, by which 

 alone many questions can be solved. 



I may say that the description I have above given of P. varians 

 is compiled solely from Cambridgeshire specimens, as the object of 



d2 



