Chapter VI —43— The Plastlds 



than 40fx. Each cell of Spirogyra contains one or more chloro- 

 plasts. Each is ribbon-shaped with slashed edges, rolled into a 

 spiral by more or less tight turns and contains numerous pyrenoids 

 lined up along its median region. Closterium contains several 

 large chloroplasts analogous to those of the Diatomaceae. They are 

 plate-shaped and occupy the marginal region of the cell. 



In the brown algae or Phaeophyceae the chlorophyll is always 

 associated in the chloroplasts with a brown pigment of the carotin- 

 oid group, fucoxanthin. Because of this pigment which masks the 

 chlorophyll, the chloroplasts are often called phaeoplasts. Among 

 these algae, the Fucales (Fucus, Pelvetia, Cystosira) enclose 

 chlorophyll in all parts of the plant except in the antherozoids. The 

 phaeoplasts appear in all the cells as numerous bodies somewhat 

 analogous to those in the phanerogams. Chlorophyll is, however, 

 formed only in very small quantities in the apical cell and in the 

 oosphere where the spindle-shaped chloroplasts are very small. 

 In the antheridium, the phaeoplasts lose their chlorophyll which 

 is replaced by a crystal of carotin^ and distribute themselves among 

 the antherozoids in such a way that each of these encloses a single 

 plastid, lacking chlorophyll, but containing instead a carotinoid 

 pigment. 



In the Laminariales, the phaeoplasts are encountered in all 

 cells equally and exist in the zoosporangia. Each zoospore con- 

 tains a typical small phaeoplast possessing a carotin crystal. 



The red algae or Rhodophyceae, form in their chloroplasts, in 

 addition to chlorophyll, a red protein pigment, phycoerythrin. This 

 is present in such abundance in most species that it masks the 

 chlorophyll. The chloroplasts then are red in color and are called 

 rhodoplasts. In the Rhodophyceae, there are grouped together 

 organisms of very different structure: the lower Rhodophyceae, 

 the Bangiales, possess an extremely simple thallus composed of one 

 type of cell. The more evolved Rhodophyceae, the Rhodymeniaceae, 

 the Delesseriaceae, on the contrary, possess a complex vegetative 

 structure with differentiated tissues — meristem, assimilating, and 

 conducting tissue. 



In the Bangiales and some Nemalionaceae, such as Chantran- 

 sia and Rhodochorton, which are very primitive red algae, the cells 

 are all similar and contain each a rhodoplast of characteristic form 

 (regular disc or star-shaped, ribbon-shaped, etc.), varying with the 

 species and provided with a pyrenoid. The chlorophyll apparatus, 

 therefore, in these algae shows the characteristics of that in the 

 inferior algae (Chlorophyceae). In other Rhodophyceae the shape 

 of the rhodoplasts is closely connected with the various parts of 



^The term stigma or eyespot is used for an orange-red body found in the zoospores of 

 most flagellate algae and in the antherozoids of many green or brown algae. The stigma arises 

 sometimes, as is the case for the antherozoids of Fucus, from a plastid which loses its 

 chlorophyll and elaborates carotin; but it also often seems to be a differentiated portion of the 

 chloroplast in the ease where the cell contains only one large chloroplast or a specialized 

 chloroplast, as in the Euglenas. This stigma plays a photostatic role in cells. The carotin 

 seems to render the cell sensitive to light and the cell is oriented in the direction of 

 greatest or least light intensity (positive or negative phototactic response). 



