38 



Genetical and Cytological Studies on Pisum 



partner in a system of four chromatids associated in pairs" (Darlington, 

 1930 6, p. 51). 



There are four ways of showing that the points of contact between 

 the chromosomes in Pisum are chiasmata. 



(1) In some cases, though only under the most favourable conditions, 

 an exchange of partner amongst the chromatids can actually be observed 

 (see Fig. 6 a, the first bivalent from the left). 



'^^•^ 



(") 



3 -4 



(c) ' 3 ^3 ^3 •2-3 4 =2 



Fig. 2. Side view of first raetaphase in (a) 724^/30, a fertile plant from a semi-sterile parent; 

 (h) .';.32V30, Early Giant rogue ; (c) 615V30, an F-^ plant from Early Giant rogue x Th. 10. 

 The number of chiasmata present is printed under each bivalent, x 4000. 



(2) The loops on either side of a point of contact are at right angles. 

 This is the mechanical result to be expected from an exchange of partner 

 and it has always been found to be associated with chiasmata. 



(.3) If the points of contact between the chromosomes are regarded 

 as chiasmata, interstitial, sub-terminal and terminal, according to their 

 position, their number and distribution agrees very well with that of the 

 known chiasmata described in Tulipa (Newton and Darlington, 1929) and 



