424 Fritsch and Rich. — Studies on the Ocmrrence and 
is unnecessary to say much more about it here. We are glad, however, 
to have this opportunity of expressing our sense of obligation to a number 
of botanists, who have with such kind perseverance and care considerably 
furthered these investigations. We are fully aware of the objections that 
can be raised against observations based on the collection of isolated 
monthly or fortnightly samples by individuals who are not in direct touch 
with the work. But apart from the fact that most of those who have 
collected for us have evidently done so conscientiously, we may point 
out that in dealing with small pieces of water like those to which we have 
confined our attention, it is very unlikely that a form, which is at all 
common, will have escaped observation ; and we have been careful not to 
draw many conclusions as to relative amounts of an Alga present without 
the existence of overwhelming evidence. Much labour has been bestowed 
on a thorough examination of each sample of Algae, particularly where 
a negative conclusion was concerned. 
The conditions, which lead to and modify the process of reproduction 
in Spirogyra (and other Algae) have been investigated by Klebs, and are 
published in his ‘ Bedingungen der Fortpflanzung, etc.’ (Jena, 1896). Klebs’s 
results are of great interest, and we shall have frequent occasion to refer 
to them in recounting our own observations, but there is one objection 
to which a large number of his experiments are open. It is doubtless 
of some value to know how diverse reagents and changes in external factors 
affect the reproduction of an algal genus, but unless they are such as are 
realized in nature, they do not contribute materially towards an under- 
standing of its biology. To our thinking work of this kind is best started 
from the other side ; that is to say, we should commence by undertaking 
careful observations on the Alga in nature, and endeavour to correlate any 
changes it shows with variations in the natural conditions of its habitat. 
The Alga in this method of investigation is left to react under the play 
of nature’s forces, and it is the work of the investigator to interpret such 
reaction in terms of external factors. When that is accomplished we must 
resort to experiment, which must ultimately settle whether the inference 
from direct observation is correct or not. 
B. The Occurrence of Spirogyra in Nature. 
In dealing with the occurrence of Spirogyra in nature we must dis- 
tinguish between a vernal and an autumnal phase, which are more or less 
sharply marked. It is in the vernal phase that reproduction mainly takes 
place, although, as we shall see, this is not without exception. Some 
species of Spirogyra (e. g. S', varians (Hass.), Kiitz., S', quadrata (Hass.), 
Petit) would appear to be (ordinarily) confined to the vernal phase and 
not to reappear again in the autumn, while others (e. g. S', rividaris , Rabh., 
S', affinis (Hass.), Petit, S.jugalis (Dillw.), Kiitz.) are present both in spring 
