110 BULLETIN OF THE BUREAU OF FISHERIES 
where rich food material has been washed from the land, the spores germinate and 
increase rapidly in numbers. The development will continue until the food supply 
is exhausted or other unfavorable conditions arise. In discussing spore formation 
in diatoms Gran (1912) stated: 
When we subsequently find the same species once more in abundance, we have every reason 
for surmising that the resting spores on the bottom were the principal source from which these 
forms have been derived. Ability to form resting spores must be of the utmost importance for 
the existence of the species in coastal waters. The chief difference between coastal seas and the 
ocean, so far as hydrographical conditions are concerned, lies in the extreme and rapid changes 
in such fundamental conditions of existence as salinity and temperature in coastal waters. Rest- 
ing spores, therefore, must be the means by which many species continue in coastal seas, not- 
withstanding the fact that there conditions of existence are favorable only for a limited portion 
of the year. The Arctic diatoms, for instance, which sometimes are to be found in the plankton 
of the Skager-Rak, are very easily affected by a rise in temperature, but their development takes 
place during the winter months from February to April, when the temperature is at its minimum. 
In the summer they are not to be seen, but their resting spores are then most probably on the 
bottom. In the same way a whole series of warmth-loving species pass through the winter as 
resting spores and are to be found along our shores only in the warmest months of summer and 
autumn. 
As in the case of the littoral pelagic fauna, the winter diatom flora throws an 
interesting light on the effect of the arm of Cape Cod on the winter forms in local 
waters. In summer the cold waters north of the cape form a barrier for southern 
neritic plankton. Samples taken by Bigelow in August, 1922, in Massachusetts 
Bay, contained the same diatoms as those which appeared in Woods Hole in greatest 
abundance in December. No doubt many of the northern diatoms are carried 
south in the summer, but the sudden rise in temperature apparently is sufficient 
to cause them to form resting cells or die. The effect of a slight change of tempera- 
ture was evident at the end of March, 1923, when the winter forms suddenly disap- 
peared. In winter, on the contrary, those carried south find a favorable climate 
with a supply of food material that has accumulated since the disappearance of the 
summer forms. Together with local winter neritic species they form a maximum 
the extent of which depends upon the supply of silicates, nitrates, etc., in the water, 
and remain until the food is exhausted or the temperature becomes unfavorable. 
Tn this way the arm of Cape Cod forms a southern barrier for northern littoral plank- 
ton only in summer and not at all times, as in the case of many benthonic species. 
If this assumption were based upon the neritic diatoms alone, it could hardly 
hold, because, combined with the evidence of the existence of diatom spores in all 
coastal waters, the factor of temperature alone could explain the condition, and trans- 
portation by currents around Cape Cod would not be necessary. However, as the 
most abundant species (Rhizosolenia alata) north of the cape in August was a truly 
oceanic form and proved to be the first to appear in large numbers at Woods Hole, 
I think it justifiable to attribute it to the currents, just as in the case of the northern 
copepods appearing about the same time which were certainly transported in that 
manner. 
None of the so-called “pulses” which Allen observed on the Pacific coast 
occurred at Woods Hole in 1922 or 1923. The seasonal curves rose and fell evenly. 
On April 3, 1913, Bigelow found the waters of Massachusetts Bay filled with dia- 
toms. These were not evenly distributed but appeared as brownish-colored bands 
