172 MISSOURI BOTANICAL GARDEN, 
of gas. Being more exposed to the sun than those much 
deeper in the water, the tougher, darker surface seemed to 
be a protective adaptation to these conditions, while the 
shorter, more compact and much-divided fronds could 
better resist the strain of the rough waves to which they 
are exposed in these higher positions. Whether the dark 
brown of the topmost plants, as opposed to the yellowish 
tinge of the lowermost ones, could be an adaptation to 
the different light intensities was not considered. The 
theory and explanation of Gaidukov (’04) and Blackman 
(704) would lend some plausibility to such a_ view. 
Although Prionitis is left by the receding tide it is not 
required to resist drying out, as is the case in Fucus and 
its associates, for Prionitis is always covered by water, even 
in the highest pools. It is likely that there is some con- 
centration of dissolved salts, due to evaporation, in such 
pools during low tide; but this, however, must be very 
slight in so short a time. 
As it appeared on further investigation, however, none 
of the above explanations seemed adequate. The more 
luxuriant, though less typical growth, was found in places 
little exposed to strong light or increased temperature, and 
the best growth, with fronds least dissected, was found in a 
small bay where the water was the warmest and the exposure 
to sunlight the greatest. But this, like the other stations 
showing the most vigorous growth, had the advantage of 
constant currents to renew the supply of CO2z and other 
foods, while affording a safe, protected position for attach- 
ment. 
The fact, also, that young plants were always found 
starting around the rim of the pools where COz would 
supposedly be abundant, but never on the bottom, where 
both oxygen and COz2 would, in all probability, be absent 
or much diminished, though affording safer points of attach- 
ment, led to the belief that the whole problem would resolve 
itself into a question of aération. The best growth was 
found at places best aérated, regardless of their position as 
to light, heat, or point of attachment, except as this might 
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