Hydrodictyon utriculatum . 631 
frequently an appearance indicative of the formation of a 
vesicle containing the dissolved substance of the grain. This 
presents often the appearance of a homogeneous layer, having 
a faint blue or grey colour surrounding the whole pyrenoid 
(Figs. 18 and 27). In it may often be seen portions of the 
partly dissolved starch-grains (Figs. 1 6 and 17). The signifi- 
cance of such a condition is not entirely clear, but it seems to 
suggest that the product of the solution of the starch may not 
be used as fast as formed. On the other hand, cases frequently 
occur where the grains, being dissolved, seem to lie imbedded 
in the protoplasm without any surrounding vesicle, and the 
pyrenoid itself, where the starch is entirely dissolved, is in 
immediate contact with the protoplasm (Fig. 25). 
The condition just described, in which the products of the 
solution of the starch appear in a vesicle surrounding the 
pyrenoid, is probably quite distinct from that shown in Figs. 7, 
8, and 9, where there is a clear space surrounding the young 
pyrenoids. Boubier’s 1 recent attempt to establish the region 
of starch-formation in this space, including it in the pyrenoid, 
is entirely unsupported so far as the facts in Hydrodictyon go. 
In no case have I found either the net-like structure or the 
delimiting, distinct membrane that Boubier described. The 
outer sharp boundary of the vesicle in these stages is not to 
be taken as a permanent membrane, for, besides being entirely 
wanting in many cases where there is no starch present, it is 
also lacking in every case of rapid starch-formation. 
Conclusions. 
If we turn now to a brief consideration of the bearing of the 
facts described in the foregoing pages upon the general pro- 
cesses of metabolism in the cell, it is of most importance to 
note that the pyrenoid is directly the seat of the processes 
resulting in the formation of starch. Both the structural and 
microchemical changes shown by differences in staining indi- 
cate that the process is an exceedingly complicated one, 
1 La membrane pyreno'idienne ; Bull, de 1 ’Herbier Boissier, tome vii, p. 451. 
