PLASTIDS 



67 



increase the sensitivity of the eyespot by concentrating the Hght at cer- 

 tain points. 



Of a different type is the eyespot in the zoospore of Cladophora 

 (Strasburger, 1900a), which appears to arise as a swelHng of the ectoplast 

 and to consist of an external pigmented layer with a lens-shaped mass of 

 hyaline substance beneath it (Fig. 30, B). In 

 Gonium and Eudorina (Mast, 1916) the lens- 

 shaped portion lies outside with the cup- 

 shaped opaque portion under it (Fig. 30, A), 

 an arrangement suggesting the primitive eyes 

 of certain animals. In neither portion has 

 any finer structure been detected. Mast has 

 shown that the orientation of the colony is 

 brought about through changes in the inten- 

 sity of the light falling upon the light-sensitive 

 substance. As the unoriented swimming 

 colony rotates on its axis, those zooids turning 

 away from the light have the hyaline portion 

 of the eyespot shaded by the opaque cup ; this 

 sudden reduction in the amount of light energy 

 received brings about an increase in the 

 activity of the flagella of those zooids, so that pj^ 30.— Eyespots of vari- 



the colony as a whole turns more directly ous types. A, zooid of Eudor- 

 ina; e, eyespot. {From Mast, 

 after Grave.) B, zoospore of 

 Cladophora. {After Strasburger, 

 1900.) C, anterior end of 

 Euglena viridis, showing eye- 



strated about the chloroplast, although it spot at surface of oesophagus, 



.I J J u Ai • 1 and near it a sweUing on one 



appears to be surrounded by a thm layer .^^^ ^^ ^j^^ fiageiium; face 

 of modified cytoplasm.^" A prevalent view view of eyespot at right, show- 

 regarding the interior of the chloroplast is '^a»e^^'i900.)^'i?,"eyespo1^of 



that it consists of a mass of somewhat denser Euglena velata. E, eyespot of 

 , „i ,1 , . I,- u J.U • 1- -J Trachelomonas volvocina, with 



cytoplasm, the Stroma, m which there IS a hpide pjg^^^^ granules and crystal- 

 carrying chlorophyll. Starch granules appear loid body. {D and E after 

 at one or more points in the stroma; hence '■"'^^''' 

 after fixation the chloroplast may appear like a hollow sphere, a 

 honeycombed mass, or a series of threads or plates separating lighter 

 regions (Fig. 31) in which starch may be demonstrated. It is claimed by 

 Zirkle that one or more such regions, or "cavities," are present before 

 starch appears in them and that they communicate with the exterior 

 through fine pores (Fig. 32). 



Because of the behavior of extruded chloroplasts in light, the reactions 

 of green tissues to enzymes, organic solvents, and stains, and the absorp- 



10 Fitting (1909), A. Meyer (1920, 1922), Zirkle (1926). Zirkle reviews early views 

 regarding plastid structure. 



toward the source of light. 



The Structure of the Chloroplast. — No 



definite visible membrane has been demon- 



