70 



PART II. ANATOMY AND HISTOLOGY. 



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shape ; they are especially numerous in the neighbourhood of the 

 nucleus. In parts of plants which, in the ordinary course, 

 eventually become exposed to light, the leucoplastids__deyelope - 

 Into chloroplastids. Conversely, when a part which is normally 

 exposed to light is kept in darkness, the chloroplastids become_ 

 replaced by leucoplastids. The essential function of the leuco- 

 plastids is to form starch-grains. 



The Chloroplastids or Chlorophyll-bodies, are of various form. 

 The characteristic feature of them is their function, which is two- 

 fold. 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- 



FIG. 39. Gronp of rod-h'ke leuco- 

 plastids, each bearing a pyramidal 

 starch-grain, collected round the nu- 

 cleus in a cell of the pseudo-bulb of an 

 Orchid (Phajus grandifolius). (x860: 

 after Schimper.) 



FIG. 40. Isolated chloroplastids with 

 starchy contents from the leaf of Funaria 

 'kygrometrica (550). a A young corpuscle ; 

 b an older one, V and V have begun to 

 divide ; c d e old corpuscles in which the 

 starchy contents fill almost the whole 

 space ; /and g after maceration in water 

 by which the substance of the corpuscle 

 has been destroyed and only the starchy 

 contents remain. (After Sachs.) 



fluence of light (see Part III.). 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 produced in the chloroplastid, it usually becomes filled with 

 starch-grains, and sometimes to such an extent that the substance 

 of the chloroplastid constitutes but the wall of a vesicle i^Fig. 40). 

 But starch-grains may be formed in a chloroplastid, as in a 

 leucoplastid, in the absence of light ; the organic substance 

 required for the building-up of the starch-grain being not produced 



