72 INTRODUCTION TO CYTOLOGY 



has not been reported. There is genetic evidence that plastid primordia 

 are transmitted by both parents in some species and by the mother alone 

 in others (Chapter XXV). 



A single interesting case from the algae may be cited. In Zygnema 

 each vegetative cell contains one nucleus and two plastids, all of which 

 divide at each vegetative cell-division. In sexual reproduction the 

 entire protoplast, with its nucleus and two plastids, passes through the 

 conjugating tube as a "male" gamete and unites with a similar complete 

 protoplast ("female" gamete) of another filament. The two nuclei fuse, 

 giving the primary nucleus of the new individual (zygospore nucleus), 

 while the two plastids contributed by the male gamete degenerate, leaving 

 the two furnished by the female gamete as the plastids of the new indi- 

 vidual (Kurssanow, 1911). A similar process occurs in Spirogyra 

 (Trondle, 1911) (Fig. 162). 



Conclusions. — With the possible exception of certain bacteria and 

 some other protista, the entire organic world is dependent for its suste- 

 nance upon plastid activity, for it is by virtue of their chloroplasts that 

 green plants are able to synthesize carbon-containing food necessary 

 to the life of both plants and animals. Many theoretical questions, 

 including that of the mechanism of heredity, involve the history of 

 plastids as individuals. So far our knowledge of this history is incom- 

 plete. In some organisms plastids can be traced with fair confidence 

 through the life cycle, but in most cases the situation is not clear. Not 

 only is it difficult to obtain satisfactory evidence for the division of the 

 smallest visible proplastids, but it is wholly impossible to determine by 

 direct cytological observation what may be their history before they have 

 passed above the limit of visibility. Hence "the question regarding the 

 extent to which the plastids are to be considered permanent cell organs 

 with an unbroken genetic continuity throughout the life cycle must 

 remain an open one" (Randolph). The whole problem is complicated 

 by the debated question of the relation of plastids to chondriosomes, a 

 topic to be discussed in Chapter VI. 



It may be remarked that cytologists have often been inclined to under- 

 estimate the capacity of protoplasm for epigenetic differentiation and so 

 have been too averse to the idea of origin de novo. If plastids represent 

 transformations of the cytoplasm resulting from the localization of certain 

 processes, they may well be expected to differentiate anew as these 

 processes begin, and to preserve varying degrees of permanence depending 

 upon the processes carried on (Harper, Pensa). Their individual 

 continuity through certain life cycles may accordingly be interpreted 

 to mean that in such forms there is a persistence of certain types of 

 physiological activity through all or nearly all the stages. In the next 

 chapter it will be of interest to compare with plastids certain cytoplasmic 

 differentiations in animals, whose metabolism differs in important 

 respects from that of plants. 



