606 MINNESOTA BOTANICAL STUDIES. 



taken from various springs in the immediate neighborhood of 

 the large one were received four days later. 



Pebble "3" was covered on the upper parts with solitary, 

 irregularly shaped mounds, 2 mm. or more in diameter. Some- 

 times these were confluent. Each thallus was capped by a 

 light- colored grain, .5 to 1 mm. in diameter, which produced 

 an odd effect. The "caps" when observed with low power 

 showed a compact structure, not flaky or uneven like the frag- 

 ments of lime which are scattered through the interior of the 

 thallus. They were perforated with numerous tubes which 

 the filaments and branches of the plant had occupied. 



Pebble "4" possessed a dark green, mucilaginous coating 

 which was not made up of solitary thalli but was continuous. 

 But for the color, it had all the appearance of an Oscillatoria. 

 It had none of the "caps" described above, but was character- 

 ized by very fine, sand-like grains which were scattered 

 throughout the substance and could be discerned with the 

 naked eye. The other pebbles and bits of glass were covered 

 with a continuous layer of cells, connected into extensive 

 plates. 



MICROSCOPIC STRUCTURE. 



1. Pilinia Stage: The microscopical appearance of the 

 alga may perhaps best be understood by describing it as a 

 Coleochaete-like plate of cells giving rise to upright Chaet- 

 ophora-like filaments. The thallus resulting from this manner 

 of growth takes the form of a flat- topped mound or a thickened 

 disc, and while in general appearance it resembles the ordinary 

 Chaetophora type, it is thus seen to differ in internal structure 

 from the latter which is made up of filaments radiating from a 

 common center. Microscopic investigation is necessarily con- 

 fined to fragments or at least to young thalli, since the cover 

 glass breaks and crushes the mass out of shape. 



The usual appearance under the cover glass consists of 

 detached rows of plate cells with the filaments standing out 

 at right angles (PI. XXXII, fig. 1). This figure will be seen 

 to be quite similar in its vegetative appearance to those of 

 Acroblaste represented by Reinsch (I) Pi. III. fig. 1, 2, 3. 

 The resemblance goes no further, however, since the repro- 

 ductive organ of Acroblaste is confined to ihe end of the 

 filament and is peculiar in structure and behavior. The 

 measurement of the filaments and microzoospores of Acro- 

 blaste also agrees with that of Pilinia in the same portions. 



