THE PLANT-CELL 



49 



only in cells exposed to the light, as their function is that of the 

 assimilation of carbon-dioxide, which can only take place in the 

 light. In the lower plants, like many Green Algae (Fig. 32, A), there 

 is but a single chloroplast in each cell. It then may have the form 

 of a cup, as in most Volvocaceae, a central axile band, as in Meso- 

 carpus, an equatorial band, as in Chsetophora, etc. In the higher 

 plants the chloroplasts are usually numerous, and most commonly 

 are oval, flat disks. The body of the chloroplast shows the same 

 spongy structure as the cytoplasm, and the green pigment in a solu- 

 ble form is supposed to occupy the spaces of the spongy body, from 

 which it can be readily extracted by means of alcohol, ether, and 

 other reagents. The colorless matrix is then left unchanged in size 

 or form. 



Fio. 32. A, three cells from the thallus of Coleochsetejicutata, showing the single 

 chromatophore and pyreuoid in each (x 600). B, leucoplast, with young starch- 

 granule, from the pseudo-bulb of Phajus grandifolius. C, leucoplast with fully 

 developed starch-granule, of the same species (X540). (B, C, after STRASBURGER.) 

 D, a cell from the pulp of a " hip" of Rosa rubiginosa, showing the orange-red 

 chromoplasts (X 250). E, cells from a ray-floret of Gazania splendens, showing 

 rounded, orange-red chromatophores (X 500). 



Pyrenoids. Associated with the chloroplastids, especially among 

 the Algae, are special bodies, Pyrenoids (Fig. 31, A), whose exact 

 nature is still not clearly settled. These very often have about them 

 an accumulation of starch which suggests that they may be concerned 

 in the process of carbon assimilation, but they have also been con- 

 sidered as themselves products of the activity of the chloroplast, and 

 to be merely masses of reserve nitrogenous food. The former view 

 is perhaps the more probable. Chloroplasts are not necessarily 

 green, as in some cases, e.g. Red and Brown Algae, accessory red or 

 brown pigments are associated with them. Even where the chloro- 

 plasts appear green, it is readily shown that in addition to the chlo- 



