MEIOSIS 203 



Wall formation in Zea mays has been investigated by Reeves. 

 Like Castetter, he also found hyaline eallose surrounding the 

 protoplasts in the young anthers. This constitutes the mother- 

 cell wall, and is thicker in regions where the protoplasts are in 

 close contact. After the completion of the first meiotie division, 

 thickenings are formed on the spindle fibres at the equator of the 

 cell. Tlie thickenings now fuse together and form the cell-plate, 

 and since they follow the formation of the peripheral spindle 

 fibres, they ultimately extend to the mother-cell wall. The cell- 

 plate splits, and a homogeneous substance appears between its 

 halves. This substance does not give the pectic reactions of a 

 normal middle lamella, but is apparently of the nature of eallose. 

 The formation of cell-plates is similar after both heterotypic and 

 homotypic divisions, and when mature, the microspores are liber- 

 ated from the eallose wall by its disintegration. In Zea mays there 

 is no quadripartition of the mother-cell by furrowing. 



MEIOSIS 



It is not proposed to discuss here the rival claims made by the 

 telo- and parasynaptic schools of thought as to the probable 

 mechanism of chromosome reduction, but only to consider peculiar 

 conditions of meiosis in different plant groups and their correla- 

 tions. 



(Enothera. The first of these that calls for consideration is the 

 genus (Enothera, which has been studied in very considerable detail 

 by Gates and his co-workers. The first publication dealing with the 

 meiotie phase in this genus w^as by Gates in 1908, who showed 

 that there was an end-to-end arrangement of the chromosomes 

 following on synapsis. This telosynaptic interpretation has 

 been followed by all subsequent investigators, with one or 

 two exceptions. In fact, it w^ould seem that (Enothera furnishes, 

 jpar excellence, an example of telosynaptic reduction. Sub- 

 sequent work has shown that in the Onagra group of (Enotheras, 

 the segmentation of the spireme takes place in a characteristic 

 manner for each species, whether it be mutant or hybrid. A 

 certain number of free pairs of chromosomes are formed and the 

 rest remain connected in a ring. 



