38 THE CELL 



Schimper amyloplasts (Starkebildner). In epidermal cells, and especially 

 in hairs, they often correspond to chloroplasts which have become 

 abortive, and sometimes seem to be functionless throughout their exist- 

 ence. In certain plants (some OKCHIDACEAE and COMMELYNACEAE), on 

 the other hand, they are so large and numerous as to arouse the 

 suspicion that they perform some special but hitherto undiscovered 

 function. 



The three different types of chromatophore enumerated above are 

 phylogenetically homologous structures, which in many cases also 

 display direct genetic relationships. Thus a large proportion of the 

 leucoplasts which occur in the embryonic cells of Phanerogams subse- 

 quently become converted into chloroplasts, while many of the latter 

 in their turn change into chromoplasts at a later stage of development. 

 A similar transformation of leucoplasts into chloroplasts may also take 

 place within permanent tissues, for instance in the peripheral paren- 

 chyma of a Potato-tuber which has been exposed to light for some time. 

 As, already stated, chloroplasts may on the other hand revert to the 

 colourless condition in certain cases, for example, in epidermal cells 

 and especially in trichomes. Schimper is probably right in assuming 

 that the chloroplast, or rather the photosynthetic chromatophore 

 generally, is the most primitive of the three known types ; from 

 the phylogenetic point of view leucoplasts and chromoplasts would 

 thus represent later developments, which arose concurrently with an 

 increase in tissue-differentiation, in accordance with the principle of 

 division of labour. 



Previous to the researches of Schmitz, Schimper and Arthur 

 Meyer, it was generally believed that chromatophores and more 

 especially chloroplasts, which were at that time the only chromato- 

 phores that had been at all adequately investigated could originate by 

 differentiation from ordinary cytoplasm, as well as by division of 

 pre-existing chromatophores. It is now almost universally admitted 

 that chromatophores never arise otherwise than by division. This 

 view was first put forward by Schmitz with reference to the chloro- 

 plasts of Algae, but has since been extended by Schimper and by 

 Meyer so as to include chromatophores of every sort. Schimper 

 was able to show that in a number of cases (roots of Azolla and 

 Lemna ; aerial roots of epiphytic Orchids ; apical cells of Mosses) the 

 cells of the apical meristem actually contain bright-green chloroplasts. 

 Where the apical meristem is devoid of chlorophyll, it is, at any rate, 

 provided with leucoplasts, which become converted into chloroplasts 

 later on ; it is an easy matter to verify the presence of these 

 leucoplasts in the apical meristems of such plants as Impatiens 

 parvijlora, Tropaeolum vwjus, and Dahlia variabilis. According to 



