3o6 Phillips on a Comparative Study of the 



frequently, the case, then the heterocyst cell will develop 

 into a spore also. The development of this spore into a 

 trichome will depend upon the amount of material passed 

 into it from the other cells. If it has formed in the middle 

 of a trichome, and can be fed as the spore cell is, it will 

 develop into a spore of as much strength as the other cell, or 

 if it has only a few cells to feed it, it will be smaller and 

 weaker. This would seem to argue that in Cylindrosper- 

 mum, at least, the heterocyst is not a dead cell as it is usu- 

 ally supposed to be. All stages in its development, as here 

 described, have been seen and it is not a difficult task to 

 reproduce the condition. Not infrequently the terminal 

 heterocyst, in material grown on a soil saturated with a 

 full culture solution, will divide once before its chromatin 

 becomes diffused, thus forming a double heterocyst (Fig. 

 56). After the formation of the spore, and the deposition 

 of the plug which closes the pores at the ends of the spore 

 cell, the chromatin which has been passing along the trich- 

 omes may not cease to flow, but become heaped up within 

 the cell next to the spore, which then begins to grow and 

 becomes a spore (Fig. 64) like the first, and with all of the 

 steps taken by it except the preliminary division which cuts 

 off the heterocyst. 



In Spermosira the spores may form in any part of the 

 trichome. One spore usually forms first in the manner 

 already described for Cylindrospermum, except that the 

 heterocyst cell is not cut off as in that organism. The 

 chromatin here does not take the form of vesicles, but is in 

 the form of more or less angular grains. After the forma- 

 tion of the first spore, others commonly form on either side 

 of it, until a long string (Fig. 64) has been developed. 



In Wollea saccata, the spore cell receives the material that 

 is passed along the trichome just as in the other forms. It 

 thus grows to twice or even three times the thickness of 

 the vegetative cell and elongates until it is six to eight times 

 the length (Figs. 84 to 86). The chromatin first becomes 



