248 p. 0. BOWER. 



It has been above stated that the pyrenoids are in many 

 cases, and especially in the green Algee^ surrounded by aggre- 

 gations of starch-grains, which sometimes fuse together so as 

 to form a starchy sheath (Amylumheerd). These starch-grains 

 and sheaths lie completely outside the pyrenoid, and are 

 embedded in the substance of the chromatophore. This shows 

 that the pyrenoid does not directly take part in the formation 

 of the starch-grains, though from the constancy of the position 

 of the latter, the idea may well be entertained that the pyre- 

 noids give rise to a soluble substance, which is transformed 

 into starch in the surrounding substance of the chroma- 

 tophore. 



The pyrenoids are multiplied in two ways : either by divi- 

 sion into two, or by a new formation. 



The division is naturally most simple in the case of the 

 pyrenoids which have no starch-sheath. As in Bryopsis 

 plumosa (fig. 15), where the stages of the process are simply 

 elongation, median constriction, and ultimate complete separa- 

 tion of the two terminal parts. A new formation of these 

 simpler forms of chromatophore has also been observed with 

 certainty in some cases, though the absolute proof of such a 

 process is a matter of considerable difficulty. They appear 

 first as very small spherical bodies of substance similar to that 

 of the mature pyrenoid, and increase gradually till the normal 

 size is attained. 



In the case of the more complicated pyrenoids with starchy 

 covering, the division is most readily followed in those exam- 

 ples where there is but one in each cell, which divides also 

 into two parts on the division of the cell (fig. 17). 



Here the division of the pyrenoid proceeds as in the above 

 simpler case; the sheath of starch-grains at first remains 

 unaltered, but later, with or without a slight previous constric- 

 tion, the sheath is completed round each of the new pyrenoids 

 by the formation of two layers of fresh starch-grains parallel to 

 the plane of division. , 



As might be expected, these processes are often complicated 

 and varied in different cases ; and as an example may be taken 



