340 Watson on Structure and Relation of the Plastid. 



Vauchcria. Oscillaria. — In the former the chlorophyll is 

 distributed in plastids of varying size in the cell protoplasm. 

 In Oscillaria the condition is still more primitive, and the 

 chlorophyll green is diffused throughout the protoplasm with 

 no attempt at localization. 



Coleochacte has a similar diffuse chlorophyll structure. The 

 nuclei stain densely, of a density similar to that of pyrenoids, 

 and in many instances show small refractive nucleoli. These 

 nuclei are situated at or very near the center of the cell and 

 are surrounded by a rather clear area. This area, however, 

 is not bounded by a membrane of any kind. 



The nuclei of Coleochacte suggest in their appearance the 

 pyrenoids of preceding plants, while their contained nucleoli 

 resemble, in their strong refraction and density, the nucleus 

 of Zygncma. 



NitcUa. — In this plant the plastids are arranged in long 

 parallel rows close beneath the cell wall and are held together 

 against the force of circulating protoplasm by branching 

 strands, which place the whole system in communication. 

 There seems to be no regularity of branching of these 

 strands, the whole merely forming an irregular network. 

 The strands take the eosin stain slightly, Intt owe their dis- 

 tinctness to their refractive character, and also to the gran- 

 ules adherent to their substance. 



The plastids are of a uniform, quite densely staining char- 

 acter and divide by simple fission. Fig. X (d) shows a 

 plastid dividing; while (s) represents two plastids just after 

 completion of the septum. 



Anthoccras. — The plastids of this plant are joined together 

 by interplastid fibres and form a general network rystem as 

 in Nitclla. The plastids are of a large, cjuite densely stain- 

 ing character, and exhibit a finely granular structure. Along 

 the interplastid fibres are distributed numbers of fine gran- 

 ules seemingly of the same nature as those within the plastid 

 (Fig. XII). 



The nuclei are of large size and their substance of a 



