40 



THE CELL 





of more or less numerous leucoplasts within the egg-cells of several 

 Phanerogams (Daphne Blagayana, Hyacinthus non-scriptus, Torenia 



asiatica. Cf. Fig. 5 a). Nothing whatever takes place, either during 

 the ripening of the seed or at its germination, to justify the supposition 

 that these leucoplasts are dissolved and replaced by newly differen- 

 tiated chromatophores. The structures in question are not, indeed, 



always readily visible during 

 this stage of the life history; 

 but the difficulty in this re- 

 spect arises from the presence 

 of large numbers of more con- 

 spicuous inclusions, such as 

 starch-grains, protein-granules, 

 or oil-drops, and all the avail- 

 able evidence argues strongly 

 in favour of the persistence 

 of the chromatophores. In 

 any case, there are no valid 

 reasons for assuming that 

 chromatophores originate de 

 novo in the developing embryo. 

 It may therefore be re- 

 garded as an established fact 

 that chromatophores like nu- 

 clei invariably arise by divi- 

 sion of pre-existing organs of 

 the same kind. The act of 

 division itself has been studied 

 in detail mainly in the case of 

 chloroplasts. Typically, chloro- 

 plasts divide by a process 

 of simple constriction, which 

 closely resembles the amitotic 

 division of nuclei ; so far as 

 is known the structure of the chloroplast undergoes no alteration 

 during this process. Fission of chloroplasts was first observed 

 by Nageli in certain Algae (Bryopsis, Valonia), in Nitclla and 

 in Fern-prothalli. Mikosch has observed a somewhat more com- 

 plicated mode of division in the cortical cells of the aerial roots of 

 Ghlorophytum comosum (Fig. 6a). Here the chloroplast exhibits a uni- 

 form green coloration in the resting condition. Division is initiated 

 by an aggregation of the pigment at two opposite poles, and by a con- 

 comitant disappearance of colour in the equatorial zone. Later, the 



Fig. 6. 



A. Chloroplasts from the cortex of the aerial root of 

 (Morophytum comosum .illustrating two modes of division ; 

 a and b, simple fission ; d-f, division with formation of a 

 colourless equatorial zone. B. Dividing chloroplasts from 

 the cortical parenchyma of the stem of Selaginella Kraus- 

 siana. C. Chains of chloroplasts from the innermost 

 cortical layer of S. Kraus.nana. D. Long chain of chloro- 

 plasts from the outer cortex. A after Mikosch. 



