300 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
thesis to take place. Though the blue-violet rays of sunlight may be 
detected at depths as great as 500 fathoms, and green light somewhat 
lower down, the red rays, the most effective photosynthetically, are 
probably all absorbed in the upper 250 fathoms. At just what depths 
photosynthetic activity may still be possible for marine plants is yet 
to be definitely determined, but surely for the diatoms it must be very 
limited to at most 50 fathoms, even under the most favorable 
circumstances. 
When the phytoplankton sinks below the level where photosyn- 
thetic activity is no longer possible, it soon perishes. Remaining 
afloat within the higher levels of the sea, therefore, becomes a matter 
of life or death to all planktonic plant life. 
Living matter is heavier than sea water. Its higher specific gravity 
must be compensated in some fashion if sinking is to be retarded. 
We find many adaptations among marine plants designed to achieve 
this end, the storage or retention within the body of lighter materials, 
such as oil droplets, various fatty substances, and even water of a 
lower salinity than sea water, and the inclusion of air or gas in 
vacuoles. Hard parts are usually drastically reduced. By the devel- 
opment of projecting horns or branches or bristlelike structures, by 
taking on a bladderlike or floatlike form, or by the forming of aggrega- 
tions, ribbons, or chains of individuals, surface area is increased and ~ 
with it the ability to remain afloat. Here again small size is of great 
advantage. Just as reduction in body size increases the ratio of sur- 
face to volume and facilitates absorption, so a high ratio of surface to 
volume retards sinking, especially in a solution as heavy and viscous 
as sea water. Some diatoms take on a slender rodlike or hairlike form 
which also facilitates floating in a horizontal position. Vertically, 
sinking would be thereby accelerated, except for a further adaptation 
to overcome this tendency. These slender forms are either a little 
curved, or have sloping or oblique ends, so that in pressing against the 
water in sinking, the diatoms are rapidly turned back to a horizontal 
position. The horns with which some dinoflagellates are provided are 
also bent in order to keep these organisms broadside on, retarding 
sinking. The dinoflagellates, as the name implies, are provided with 
a pair of flagella with which the plant can propel itself after a fashion. 
Most coccolithophorids are also flagellated, motile forms. 
The various floating forms of plant life are found in incredible 
numbers over wide expanses of all oceans and form the so-called 
meadows of the sea. Not in kind, but in the manner of their culture, 
they correspond, for the purposes of our discussion, to the plants 
grown hydroponically. 
From time to time investigators have made estimates or contributed 
remarks regarding the abundance of the phytoplankton in areas with 
