98 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



modified and associated in the organ. The behavior of the 

 microsjTnbiont in this symbiosis is also significant and will 

 be discussed presently. 



We have followed some of the responses that take place 

 in the relative symbiont, or the host, in symbiosis, we have 

 next to consider the modifications that arise in the definitive 

 symbiont. These are the more difficult to analyze, but 

 they form the "connecting link" in the theory of Symbion- 

 ticism. It is in this connection that the responses shown 

 by algal symbionts furnish important evidence. 



Keeble and other investigators have described the modi- 

 fications that algae undergo in the symbiotic situation. 

 In the first place, some of the algal species that enter into 

 symbiosis are flagellate in the free-hving condition. These 

 flagella are lost when the algae enter the tissues of the host. 

 Some species, after they have taken up the symbiotic 

 position undergo further modifications; the cell wall, nu- 

 cleus, and pyrenoid disappear, so that all that remain of the 

 original algal cell are the chloroplasts. These chloroplasts, 

 apparently, continue to perform their normal functions, 

 for starch granules can be demonstrated by the iodine 

 reaction in the cytoplasm of the host cell. 



We may then inquire, — what is the nature of the chloro- 

 plast? It has been looked upon as an ''organ" in the 

 plant cell. Just what is meant by an intracellular ''organ" 

 is difficult to determine from its usage in the Uterature. 

 In general, it appears that the conception of cytoplasmic 

 "organ" has carried with it the idea of a passive or non- 

 living body. Such a conception is difficult to correlate 

 with what we know of the activities of chloroplasts. We 

 might further ask. Is the chloroplast only a "container" 

 for the chlorophyl, and is the chlorophyl only a sort of 

 "catalyzer?" We can extract chlorophyl from plants, 

 mix it with carbon-dioxide and water and place the mix- 

 ture in sunUght, but no starch is produced. There appears 



