196 BOTANICAL GAZETTE | SEPTEMBER 
section near their mouths too salt for the growth of Isoetes, and 
this salt water section of the rivers becomes longer as we pro- 
ceed toward the mouth of the bay. This isolates the suitable 
habitats in one river from those in neighboring rivers. 
c) There must be sufficient stability of the soil of the shore 
to resist the action of the waves, and at the same time sufficient 
fineness of the soil particles to supply the needs of the plant 
without requiring an extensive root system. With one excep- 
tion all the colonies visited by me grew on beaches characterized 
by rather coarse gravel set firmly in a matrix of sand, and cov- 
ered over with a thin film of mud. The exception was found at 
my upper station on Gunpowder River, where a small but unusu- 
ally luxuriant colony grew ina bed of sand. In this place the 
shore was protected from severe wave action by a zone of Zzzanta 
aquatica L., and I have no doubt that this circumstance alone 
made it possible for Isoetes to retain its hold at this place. 
In Engelmann’s manuscript notes, the following statement is 
accredited to William M. Canby: “I don’t find any Isoetes 
(riparia, Engelmannt, valida, or saccharata) in pure mud or pure 
gravel; they always grow in mud which is deposited on gravel 
beds either by the tides (riparia and saccharata) or by rains which 
wash it down (Angelmanni and valida).” 
ad) Isoetes saccharata also requires that competition with other 
plants be slight. It is never found forming colonies of such 
density that it crowds itself, and its most frequent companion 
species have the same scattered habit. Only at my lower station 
on Gunpowder River have I found an exception tothis. Here it 
is in competition with Scirpus americanus Pers. and is evidently 
suffering inthe conflict. It would no doubt be entirely excluded 
by the Scirpus if the latter were as robust and densely set as is 
usual for that species on fresh water beaches. 
When we consider the number of apparently essential elements 
in its habitat and the fewness and smallness of the areas in which 
all these elements are present, it is easy to understand the 
extreme localization of the species. But every restriction of 
habitat increases the difficulty of successful dispersal, and we may 
well ask how the species has succeeded in finding the places, often 
