22] 



CHAPTER I. THE CELL. 



71 



in the chloroplastid itselfj but supplied from other parts of the 

 plant. 



These plastids are termed chloroplastids, because the colouring- 

 matter upon which their assimilatory function depends is most 

 commonly the familiar green colouring-matter, chlorophyll. But 

 thy_arejioXalways green. In some of the Algae they are red or 

 brown, because in addition to chlorophyll there is present in the 

 one case (Rhodophyceae), a red colouring- matter, phycoerythrin, 

 and in the other (Phseophyceae) a brown colouring-matter, 

 phycoxanthin or phycophcein. These substances are, however, 

 related to chlorophyll. 



When the colouring-matter is dissolved out by alcohol or some 

 other solvent, the protoplasmic plastid is left colourless, but un- 

 changed in form or size. Thej^hlorpphyll appears to exist in an 

 ojlv__splutipnj and to be con- 

 fined to the fibrillar portions 

 of the plastid, in the form of 

 draplets (#ro2ia). 



The most common form of 

 chloroplastid the only one oc- 

 curring in the higher plants 

 is the chlorophyll -corpuscle 

 (Fig! 40), which is flattened 

 and discoid. Usually, many 

 corpuscles are present in a cell, 

 but occasionally (e.g. Antho- 

 ceros) there is only one. In 

 the Algse the chromatophores, 

 though sometimes small and discoid (e.g. Vaucheria, Fucus, etc.), 

 are more commonly large, occurring singly, and of very various 

 form. 



The chromatophores of the Algse present a great variety of form. 

 Generally speaking, those of the higher forms are small corpuscles of a 

 more or less discoid form ; while in the lower forms the chromatophores 

 are few in number, often single, in each cell, and are relatively large, 

 assuming commonly the shape of a flattened plate, sometimes elongated 

 and straight or spirally coiled (Fig. 41). In the latter case the large 

 flattened chromatophores present one or more spherical thickenings, each 

 of which is termed a gjtrenoid (Fig. 41), and consists of a homogenous 

 colourless mass of proteid substance. The pyrenoid is generally sur- 

 rounded by a layer of starch-grains : this is, in fact, the only part of the 

 chromatophore in which starch can be detected. 



FIG. 41. Spirogyra majuscuZa (after Stras- 

 burger: x2VO). A cell of a filament, showing 

 the nucleus suspended in the centre ; also 

 the spirally-wound chromatophore with py- 

 renoids. 



