1 1 6 Chodat. — On the Polymorphism of the Green 
It is not necessary, however, to have recourse to cultures in 
nutritive solutions ; in wet years all these developments can 
be observed on the bark of shady trees exposed to rain and 
moisture. I have renewed these experiments by starting 
with Pleurococcus gathered in various localities in Switzerland 
and France, and have always arrived at the same result. 
A second variation, which is quite as frequent, is the pro- 
duction of spores. In cultures or in fresh conditions this 
takes place sometimes by the gradual solution of the 
separating cell-walls of the thallus. During this process 
the cell-contents acquire a special envelope and become more 
and more globular. The general envelope of the thallus is 
not altered, but assumes the function of a sporangium-wall. 
In this case it may be that on a large thallus one half of the 
cells undergo this transformation, whilst the remainder retain 
their normal structure. Some of the cells of these small 
typical thalli may grow larger and become more globular ; 
at length the latter are separated from the normal cells and 
constitute very different bodies, in which division of the 
contents generally takes place very soon, and spores are 
formed in this way. This production of sporangia is of 
frequent occurrence in connexion with the normal Pleurococcus 
cluster of cells, in which I have often observed not only the 
above- described sporangia but also at the same time more 
or less developed filaments. This shows very obviously how 
difficult it is, with regard to this and other productions, to 
discover the true cause of their formation, when they grow 
not only under the same conditions but even on the same 
plant. 
Finally, each cell of the more or less complicated cluster 
of Pleurococcus can be renovated by a similar process as I have 
just described above for the formation of spores from the 
vegetative thallus ; but in this case the renewed cells retain 
the same elongated or more or less quadrangular form, and 
are consequently very like some Stichococcus. But this is 
only an external resemblance. Stichococcus is quite a different 
plant, belonging to the Ulothricaceae. All these sporangium- 
