192 BOTANY 



life-history is followed by fusion in pairs of nuclei of opposite sex 

 during this stage. No evidence, however, is brought forward in 

 support of this statement. 



At the conclusion of the transitional period the reconstructed 

 nuclei show a well-developed aster and centrosonie, but the 

 nucleolus is inconspicuous. These nuclei are considered to be 

 diploid in nature. Meiosis now occurs ; the first division is held 

 to be heterotypic, and the chromosome number is reduced accord- 

 ing to the telosynaptic scheme of Farmer. A homotypic follows 

 the heterotypic division, and occasionally there is a third division 

 which immediately precedes spore formation. 



Cyanophyceae. The cell structure of the Cyanophyceae, or blue- 

 green algae, has always been a subject of much dispute. It has been 

 known for a considerable time that there is in the cells of these 

 plants a central region which is relatively colourless in comparison 

 with the rest of the cell contents. It is the nature of this central 

 body which has led to such acute differences of opinion. The 

 more general view is to regard it as corresponding to a nucleus, 

 but its nuclear nature has also been extensively denied. According 

 to Kohl, Olive and Phillips, the central body of the Cyanophycean 

 cell is a nucleus which divides mitoticallv. But these workers 

 differed materially among themselves as to the arrangement of 

 the chromatin and the actual details of division. Fischer, on the 

 other hand, denied the presence of a nucleus in these cells, while 

 Gardner held that the small refractive granules which he found 

 in the cells of these plants possessed a definite outline charac- 

 teristic for each species and regarded this as the true nucleus, but 

 considered that it did not divide mitotically. 



Acton has investigated the nature of the central body very fully 

 for the Chroococcaceae. She found in this familv that there was 

 no highly specialised nucleus such as is present in the higher 

 plants. There was, however, a gradual transition in the cells from 

 an almost undifferentiated condition in the lower types to a some- 

 what specialised one in the higher, of which Chroococcus macro- 

 coccus represented the highest type and Merismojpedia elegans an 

 intermediate stage. The protoplasts consisted of a ground sub- 

 stance transversed by a reticulum of thin threads with thickenings 



