114 NOTES ON PONDWEEDS. 
rather than elucidate the real differences between P. Zizii and the 
plants nearly allied to it. 
Fro heterophyllus, which it closely resembles in habit of 
growth, it may usually be distinguished by its larger size, and b 
the coriaceous floating leaves being less thick and being more 
gradually attenuated into the petiole. The drupelets are larger 
than those of P. heterophyllus, and smaller than those of P. lucens ; 
in shape they usually resemble fruit of the former species, but some . 
deep-water forms of P. Zizii produce fruit very like that of typical 
lucens. Apart from degrees of maturity, the fruit of P. Zizii is too 
variable to admit of exact description, a fact which perhaps indi- 
cates the need of further specific division. 
Closely as this species resembles P. Jucens on the one hand, and 
P. heterophylius on the other, and closely as the three species follow 
one another in their many variations, they never seem actually to 
are probably mere states dependent on local conditions. The 
mperature. In nearly dried-up waters all the leaves, lower as well 
as upper, become much thickened in texture, and the truly floating 
leaves often completely cover the surface, being then abundantly 
produced on the barren as well as on the fruiting branches. Their 
thickened substance enables the leaves to sustain the direct rays of 
the sun, and the dense covering of foliage hinders the evaporation 
of the already wasting pools. In the warm black soil of the fens 
