98 



PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 23. 



The Leucoplastids may be spheroidal, fusiform, or cylindrical in 

 shape ; they are especially numerous in the neighbourhood of the 



nucleus. In parts of 

 plants which, in the or- 

 dinary course, eventually 

 become exposed to light, 

 the leucoplastids de- 

 velope into chloroplas- 

 tids. Conversely, when 

 a part which is normally 

 exposed to light is kept 

 in darkness, the chloro- 

 pListids become replaced 

 by leucoplastids. The 

 essential function of the 

 leucoplastids is to form 

 starch-grains. 



The Chloroplastids or 

 Chlorophyll-bodies, are of 

 various form. The cha- 

 racteristic feature of 

 them is their function, 

 which is twofold. In 

 the first place, they can, 

 like the leucoplastids, generally produce starch-grains ; in the 

 second place, they are capable, in virtue of the colouring-matter 

 present in them, of constructing organic substance from carbon 



dioxide and water under the in- 

 fluence of light (see Part IV.). 

 Their function is thus not only 

 starch-forming or amyloplastic, but 

 also assimilatory. These two 

 functions may be, and usually are, 

 carried on simultaneously ; hence 

 when, under the influence of light, 

 organic substance is being pro- 

 duced in the chloroplastid, it 



Fia. 61. Group of rod-like leuco- n i_ nn j -11 



plastids, each bearing a pyramidal USUa % ^COmes filled With starch- 



starch-grain, collected round the nu- grains, and sometimes to such an 

 deu 8 ina C ell fthep8eudo.bulbof a n exfcenfc th t th gubstance of the 

 Orchid (PTiojus grandifohus) . (x850: 



after Schimper.) chloroplastid constitutes but the 



FIG. 50. Chloroplastids in the cytoplasm of the 

 cells of the prothallium of a Fern. A Optical section 

 of the cells ; B part of a cell seen from the surface. 

 Some of the plastids have begun to divide ( x 400) . 



