PLASTIDS AND CHONDRIOSOMES 113 



between the two seems highly probable ; whereas in other cases (Gonium, 

 Cladophora) it appears to represent a differentiation of the ectoplast. 

 It is more than likely that light-sensitive organs have arisen more than 

 once in the evolution of the lower organisms, and that they cannot all 

 be placed in the same category. 



The Individuality of the Plastid. It was believed by the early 

 observers, notably Schimper (1883) and Meyer (1883), that plastids 

 never originate de novo but always arise from preexisting plastids 

 by division. Fully differentiated plastids, such as chloroplasts, can 

 readily be seen multiplying in this manner in growing tissues with a 

 frequency sufficient to account for the large number of plastids present 

 in mature plant parts. Since it is known, however, that chloroplasts 

 and other differentiated chromoplasts may arise from leucoplasts through 

 the development of pigments and other characters in the latter, and also 

 that the individual plant arises from sex cells or a spore in which the 

 plastids are usually in a colorless and relatively undifferentiated state, the 

 problem of the individuality of the plastid is mainly one of determining 

 whether these undifferentiated plastids, leucoplasts, or "plastid primor- 

 dia" later developing into chloroplasts and other types are continuous 

 through the critical stages of the life cycle, multiplying only by division, 

 or arise de novo as new differentiations of the cytoplasm. At this point 

 we may review certain cases in which the plastid has been followed 

 through gametogenesis and fertilization. 



In Zygnema (Kurssanow 1911) 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 contrib- 

 uted by the "male" gamete degenerate, leaving the two furnished by 

 the "female" gamete as the plastids of the new individual. 



In Coleochcete (Allen 1905) each vegetative cell and gamete has one 

 nucleus and one plastid: after the sexual union of the gamete nuclei the 

 fertilized egg therefore contains one nucleus and two plastids. These two 

 plastids divide at the first division of the fertilized egg but not at the 

 second, the four resulting cells consequently having one plastid each. 

 In the third cell-division the plastids also divide, so that each cell of 

 the several-celled structure developing from the fertilized egg has its 

 single plastid. Each of the several cells eventually becomes a zoospore 

 which germinates to produce a new Coleochcete body with a single plastid 

 in each cell, the plastid dividing with the nucleus at each cell-division. 



A somewhat similar regularity in the behavior of the plastid is shown 

 in Anthoceros (Davis 1899; Scherrer 1914). Each gametophytic cell 



