624 Timber lake, — St arch- Formation in 
in many like those in Hydrodictyon, It may not be without 
significance that whereas there is no differentiated body here 
which can be distinguished as a chromatophore, still there 
are well-defined centres (the pyrenoids) for the formation of 
starch. 
If we turn now to the details of starch-formation it is to 
be noted that the whole process can be traced from certain 
structural changes occurring in the body of the pyrenoid. 
This body, while it is not undergoing such transformations, 
presents a deep red homogeneous appearance in sections 
treated with the safranin gentian-orange mixture. In outline 
it may present many variations in form depending largely 
upon its relation to the process of starch-formation as described 
below. The typical appearance, however, of the resting 
pyrenoid is, perhaps, that described by Schmitz, in which the 
pyrenoid shows a spherical form with a clear outline. In this 
condition it is often surrounded by a single layer of starch 
made of separate grains of a decidedly concavo-convex form 
(Fig. 21). This is a condition that is very often observed 
and forms the basis of most of the familiar descriptions of the 
pyrenoid. That it is a resting condition, however, is abun- 
dantly shown by the fact that during the formation of starch 
a complete series of stages may be found, leading from it 
to the conditions in which starch-formation seems to be most 
active, and by the further fact that it generally appears prior 
to the formation of swarm-spores, when starch-formation has 
ceased and the processes leading to the disappearance of both 
starch and pyrenoids have set in. The first indication of the 
changes leading to the formation of starch consists of a 
differentiation of the body of the pyrenoid into two portions, 
one of which is destined to become transformed into a starch- 
grain and the other to remain unchanged. The part that is 
to form the starch soon stains less densely. Instead of being 
stained red it now becomes a neutral grey, or faint orange. 
The portion so differentiated may include nearly half of the 
pyrenoid, but frequently does not include so much (Figs. 1-4 
and 6-8). In connexion with the change in staining capacity, 
