Algae. 539 
1. The different size-groups are not seasonal variations, but are 
regulated by the laws of growth of the Diatoms. 
2. To the study of those laws Statistical investigations are excellent, 
but they must be carried out over a long period of years, at least 5 
years, and owing to the fact that Wesenberg-Lund has never had 
continuous material for more than 3 years at his disposal, the 
results may be said to be not fully conclusive. 
3. At any particular time most of the individuals of a plankton 
Diatom are (oftenest) of one fixed size, as is proved by the steep- 
ness of the curves. 
4. The ränge of Variation in each species is of course fixed. 
The modes of the curves never reach the extreme limits of this 
ränge, but in course of the investigation we see them move slowly 
and evenly from right to left — i. e. the diminution of the size of 
the cells by cell-division; on the other hand the displacement of the 
modes from left to right proceeds suddenly and irregularly — i. e. 
the appearance of new conditions causes a considerable increase in 
size of the great majority of the individuals. This last phenomenon 
is that which Schroeter and Vogler have explained as due to 
new varieties, Bachmann as due to auxospore formation. Wesen- 
berg-Lund believes in the latter explanation, altliough he has not 
succeeded in finding the auxospores in* any of the species inves- 
tigated, but he puts forwards several observations and considerations 
which strengthen his opinion, but which cannot, owing to restricted 
space, be given here. 
5. The Diatoms occuring in plankton samples are to be divided 
into three groups: the tycholimnetic, the neritic and the limnetic 
group. The tycholimnetic Diatoms are species which really belong 
to the bottom and the littoral region, but which by rivers and 
waves are carried out into the pelagic region where they vegetate 
for a short time and then sink to the bottom; they have no distinct 
maximum. 
With regard to the neritic Diatoms in freshwater it appears 
from the investigations that a regulär change takes place between 
a fixed bottom and littoral stage and a pelagic one; the latter occurs 
during the spring, and the species have at that time a distinct, 
oftenest short, maximum. Probably the waves etc. carry the species 
out into the pelagic region; the floating power is in some species 
(Suvivelloideae ) conditioned by the perfection of their raphe appa- 
ratus (a kind of swimming movement); others ( Synedrae, Diatomci, 
Tabellariae, Stephanodiscus ) adapt themselves to the pelagic circum- 
stances by forming colonies or. if they also as littoral forms occur 
in colonies, by changing the form of the colonies from zigzag-lines 
to stars. For these species we may assume seasonal Variation de- 
pending upon the variations of the outer conditions. 
To the limnetic group belong only Asterionella, Fvagilciria 
crotonensis, Melosirae, Attheya and Rhizosolenia. They ( Melosirae 
excepted) are rather thinshelled species which are not or only slightly 
connected with the littoral region. They occur at all times in the 
water, but have well-marked maxima, often two in the year (spring 
and autumn); with exception of Attheya and Rhisosolenia they form 
colonies. We have no means for understanding how the large 
maxima are formed and in what condition the species survive the 
\ minimum period. Probably they must have some resting stages 
and in Melosirae, Attheya and Rhizosolenia resting spores have been 
observed —, but we have not the slightest knowledge if the resting 
