Chloroplast forming a sphere in Ceratophyllum demersum

[oo98ii]

Here are two light micrographs of the leaf of Ceratophyllum demersum L. In each cell, there is a sphere with several chloroplasts sticking to the inner surface of it. What is this sphere?

[MrMistery]

probably the central vacuole. I have no idea how chloroplasts might attach to the vacuole, but it's the only alternative I can think of.