226 PIGMENTS 



chloroplasts once had a separate individuality, and that, in 

 a sense, ordinary plants are parasitic upon the imprisoned 

 plastids which have become permanent members of the 

 structures of the cell. 



On the other hand, other investigators hold that the 

 chloroplasts may arise from differentiated parts of the proto- 

 plasm, which parts are not plastids. Lewitski* draws atten- 

 tion to the presence of minute bodies occurring in the proto- 

 plasm, but not in the nucleus, which he calls mitochondria, 

 chondriosomes, etc. These structures, which he considers are 

 essential parts of the cytoplasm, increase by division, and give 

 origin to the plastids. For instance in the pea, Pisum sativum, 

 and the asparagus, Asparagus officinale, the mitochondria of 

 the cells of the stem apex give rise to chloroplasts, whilst 

 those of the apex of the root are converted into leucoplasts. 

 Meyer, -f however, is opposed to these conclusions. Miller + 

 finds that very minute chloroplasts occur in the cotyledons of 

 the seed of Helianthus aniiuus, and increase in size and divide 

 by fission as germination proceeds and maturity is reached. 



In green plants chlorophyll may occur not only wherever 

 light gains access to the living cells, but also in places where 

 light seemingly cannot penetrate, at any rate in any quantity, 

 for instance in the cortex internal to the periderm — not only 

 in small twigs, but also of larger branches — in the medullary 

 rays and even in the pith.§ Also it may occur in the cotyle- 

 dons of seeds before they are set free from the ovary or from 

 the cone ; Pinus, Euonymus europcsus, and species of Cucur- 

 bita are familiar examples. In some of these cases light no 

 doubt does penetrate through the walls of the superposed cells ; 

 this may be well seen if the seeds be removed and the lumen 

 of the fruit of the vegetable marrow be cleaned out. It is 

 hardly necessary to remark that if the chlorophyll in these 

 deeply-seated tissues be functional, its contributions to the 



* Lewitski: " Ber. deut. bot. Gesells. ," 1910, 28, 538. 



fMeyer: jrf., igii, 29, 15S. See also Schmidt: "Prog. Rei. Bot.," 1912, 

 4, 163; Forenbacher: "Ber. deut. bot. Gesells.," 1911, 29, 648; Woycicki, 

 "Sitz. Warschauer Ges. Wiss.," 1912, 5, 167; and Lowschin : "Bar deut. 

 bot. Gesells.," 1913, 31, 203. 



X Miller : " Ann. Bot.," 1910, 24, 693. An excellent resume of the literature 

 on mitochondria is given by Cavers in " New Phyt.," 1914, 13, 96, 170. 



§ See Scott : " Ann. Bot.," 1907, 21, 437. 



