N. H. COWDRY. 



Physiology. 



It is generally conceded that mitochondria in plant cells play 

 an important part in the elaboration of chlorophyll and starch. 

 They are thought to do this through the intermediary of the 

 chloroplasts. According to Guilliermond ('12, p. 387) chloro- 

 phyll, appears in typical mitochondria, increases in amount, 

 other changes take place and the mature chloroplast results. 

 It is not surprising, therefore, that mitochondria have attracted 

 so much attention among European botanists, because the forma- 

 tion of these substances had been under discussion for nearly 

 a century and a deadlock had been reached before these new 

 mitochondrial methods were devised. Indeed the formation in- 

 directly of starch from atmospheric carbon dioxide and water, 

 through the action of sunlight on chlorophyll, is the most funda- 

 mental of all vital processes in plants. Mitochondria are con- 

 cerned in the formation of chlorophyll and thus the very existence 

 of the plant depends on them. Plants furnish the food of animals 

 so that the importance of mitochondria with respect to the food 

 supply is apparent. 



Very obviously the mitochondria in typical animal cells can 

 take part in no such process since there are no plastids, and 

 starch is elaborated only in plants. But this is not, as it might 

 seem to be, a fundamental distinction between the two, for many 

 consider the animal mitochondria themselves to be plast-like 

 and to act as such in the elaboration of secretions. The evidence 

 for this, however, is not entirely conclusive and we must bear 

 in mind, in all experimental work, the great importance of the 

 homogeneous ground substance, or environment, in which the 

 mitochondria are embedded. Apparent alterations may occur 

 in the mitochondria which cannot be attributed to the mito- 

 chondria themselves, but only to changes in their environment. 

 This is particularly true in the case of their refractive index. 

 The mitochondria may stand out sharply in one cell and be 

 quite invisible in another and yet be identical as far as their 

 function is concerned in the two cells, for variations in the 

 surrounding protoplasm may alone be responsible for the dif- 

 ference in appearance. There are thus two variables, the mito- 

 chondria and the protoplasm, either one of which may bring 



