68 MINNESOTA BOTANICAL STUDIES. 



one mm., the shortest of barely appreciable length by one half 

 mm. Here as elsewhere the organs are but slightly differen- 

 tiated and vary in size, structure, and number, according to 

 external conditions (PL XV., Figs. 15 and 16). 



CELL CONTENTS. 



Endochrome. The peripheral layer of cell protoplasm con- 

 sists of dove-tailed polygonal chromatophores, plate-like and 

 distinctly separated by colorless thread-like lines. This single 

 layer of thin wall plates in outer cells of thallus, is dense, stains 

 deeply, and forms a compact unbroken layer. The attachment 

 of this to the cell wall does not appear a close one as the layer 

 becomes easily detached, and floats separately in water (PL 

 XV., Fig. 77). The layer (PL XV., Fig. 77) becomes irregu- 

 larly perforated, less solid and finally (PL XV., Fig. 18) in inner 

 cells, consists of widely separated chromatophores, joined by 

 numerous thin granular threads. The endochrome does not 

 project into the cell cavity. 



In central cells, few chromatophores are present. One kind 

 of chromatophore only can be distinguished, though the size 

 and shape vary somewhat. 



Pyrenoid. Centrally placed within each chromatophore, is 

 an irregularly spherical body, with thicker walls, and stronger 

 refraction (PL XV., Fig. 18). Fig. 18 shows in section the 

 thickened wall and hollow center, but does not show the central 

 grain or clefts which are present. 



The development of the pyrenoid can be clearly traced. In 

 early stages (PL XV., Fig. 19) it is more spherical and solid, 

 except for the beginning of a cleft from the outer edge, which 

 cuts to the central grain. This latter enlarges and becomes more 

 irregular in outline, later. More divisions occur, in one plane 

 only, and four to six lobes are formed, all converging to the 

 inner grain, beyond whose outer edge the cleft does not continue. 

 This latter is spherical, thick-walled, refractive, and contains a 

 minute central grain (PL XV., Fig. 20). Acid fuchsin differ- 

 entiates the pyrenoid and accompanying starch grains clearly. 



Starch grains. A weakened iodine solution give this char- 

 acteristic starch reaction. The starch grains show concentric 

 layering and vary in shape from oval and spherical to irregular 

 shapes. They are scattered in older stages near the pyrenoids 

 or throughout the chromatophore (PL XV., Fig. 20). In 



