THE CHLOEOPHYLL APPARATUS 159 



plasm of the cell (fig. 87). Even in the lowly forms it is 

 apparently never uniformly distributed through the body 

 of the protoplast. The form, dimensions, and structure of 

 the chloroplast differ considerably in the different groups of 

 plants. In some of the filamentous green seaweeds it may 

 appear as variously shaped bands or plates. Spirogyra 

 shows it as a spiral band passing round the cell ; in Zyg- 

 nema it has the form of two star-shaped masses which are 

 attached to the cytoplasm by bridles ex- 

 tending to the cell- wall. In the brown 

 and red seaweeds the plastids are not 

 green, but have the appropriate colours 

 of the plants. These plastids contain 

 other pigments in addition to the chloro- 

 phyll, but the latter can be made ap- 

 parent by extracting the cells with cold 

 distilled' water, in which the other pig- 

 ments are soluble. In all plants higher in T P L ASM OF A CELL OF 



THE PALISADE TISSUE 



the scale than the Algae the chloroplasts OF A LEAF. 

 are found as round or oval bodies embedded in the cytoplasm. 

 They never occur in the vacuoles of the cells. Though 

 normally green, they can assume other colours, such as 

 yellow, brown, or red, but this is due to an alteration of 

 the pigment they contain. Examples of this change are 

 afforded by the assumption of the autumnal tints by 

 foliage leaves, and by the changes in colour which are 

 characteristic of ripening fruits. 



In the Mosses the chloroplasts are found throughout 

 the cells of the leaves, in the outer parts of the sporo- 

 goniurn, and in certain cells of the axis. In the Ferns they 

 occur chiefly in the leaves, occupying the cells of the 

 epidermis as well as those of the mesophyll. In the 

 higher plants they are found most prominently in the 

 mesophyll of the leaves, the epidermis as a rule being free 

 from them. When the leaves are dorsiventral in structure, 

 the chloroplasts are more numerous in the palisade paren- 

 chyma which lies just below the upper epidermis than they 



