16 



Myxophycese 



described protoplasmic connections in species of Nostoc and Anab&na, and 



Nadson ('95) figured them in Aphanizomenon 

 and Tolypothrix. Phillips ('04) stated that 

 every multicellular blue-green organism studied 

 by him was found to have intercellular proto- 

 plasmic continuity, although the protoplasmic 

 connections were difficult to demonstrate in 

 Oscillatoria. He considered that the fine proto- 

 plasmic threads passed through communicating 

 pores in the cell-walls (fig. 10), and that there 

 was usually one central pore, although other finer 

 pores were sometimes present. The protoplasmic 

 continuity described by Fritsch ('04) as occurring 

 between the cells of Anahsena, was subsequently 

 stated by that author ('05) to have been an error 

 of interpretation, and that the connecting threads 

 consisted only of the intercellular portion of the 

 investment. This may also be true with regard 

 to the supposed protoplasmic connections between 

 the cells of Nostoc and other genera. Gardner 

 ('06) entirely failed to demonstrate any proto- 

 plasmic continuity between the cells in the various 

 forms he examined. 



In certain species of Stigonema the proto- 

 plasmic connections are rather more conspicuous 

 than in other blue-green forms, a condition being 

 presented which is precisely similar to the proto- 

 plasmic continuity exhibited by the Rhodophyceas 

 (W. & G. S. W., '97 A, p. 242; G. S. W., '04). 

 The continuity is effected by a single protoplasmic 

 strand connecting the polar extremities of adja- 

 cent cells. The strands are by no means small 

 and they pass through centrally-placed pores in 

 the transverse walls of the filaments. The pores 

 are best seen in the young branches of Stigonema 

 ocellatum, particularly if plants previously dried 

 are soaked in water, or if the protoplasts are 

 caused to shrink by means of reagents. 



Fig. 10. End of trichome of 

 Cylindrospermum sp. showing 

 the terminal heterocyst and 

 spore furnished with hair-like 

 processes, and the gelatinous 

 sheath frayed around the 

 heterocyst. The protoplas- 

 mic connections are also 

 shown through the commu- 

 nicating pores in the end- 

 walls of the cells. Very highly 

 magnified (after Phillips). 



HETEROCYSTS. In the filamentous families of the Nostocaceae, Scytone- 

 maceae, Stigonemacese, and Rivulariacese, there are certain differentiated 

 cells known as heterocysts. They may be sparsely scattered in an intercalary 



