1917] BOURQUIN—ZYGNEMA 427 
plastids in which no starch is formed. He thinks that the pyrenoid 
is homologous with the protein aleurone crystals of the lower mono- 
cotyledons, and he places the pyrenoid substance in the same 
chemical group as chromatin. 
SCHIMPER (5, 6) describes the pyrenoid as a crystalline sub- 
stance which rises de novo in the cytoplasm, and which in turn 
gives rise to the sheath of starch grains surrounding it. The 
plastids of algae always form it in the process of starch formation. 
If abundant, it crystallizes; if thin, it passes into starch without 
crystallizing and its presence cannot be demonstrated by the micro- 
scope, as is possible when it is in crystalline form. It and its sur- 
rounding sheath of starch grains are structures peculiar to algae 
and not homologous with any structures found elsewhere in plants. 
He believes that it does not belong to the same group as chromatin. 
According to TIMBERLAKE (8) there is no differentiated chro- 
matophore in Hydrodictyon utriculatum, but the chlorophyll is 
distributed through the whole peripheral protoplasmic layer of 
the cell (p. 623). Just before zoospore formation the pyrenoids 
disappear. They rise de novo when the zoospores germinate. The 
substance of the pyrenoid is changed into starch grains by “pro- 
cesses not understood.” He is inclined to believe that the pyrenoid 
is an active independent cell organ whose function is to produce 
starch, 
YAMANOUCHI (9g) finds a plastid in Hydrodictyon ean a 
plant like H. utriculatum in all essential features. In its early 
stages the plastid is denser in the outer regions than in the center 
and is irregular platelike or spindle-like in form. It may produce 
either reserve starch grains or ‘chee The two are not recorded 
in the same plastid. 
CHMILEWsKY (1) believed that the starch grains in Zygnema 
are formed wholly from the substance of the pyrenoid, plates of 
which extend between the starch grains, giving a starlike structure. 
Miss Merriman (2) figures the starch grains as entirely sur- 
rounded by cytoplasm except on the side abutting on the pyrenoid, 
Since this work of Miss MErrm™aAn’s has been accepted as the 
standard on nu¢lear division in Zygnema, it seems necessary to 
emphasize the fact that she makes no claim of having interpreted 
