26 Genetical and Cytological Studies on Pisum 



note: "They comprise tiie products of individual plants in two groups, 

 Thibet 7 -and Thibet 10. The progenitor of each group was a single plant 

 in 1924. Violent colour fluctuations occur from plant to plant. The form 

 is very dwarf and early." In noting the colour fluctuations Prof. Bngledow 

 referred apparently to the distribution of purple pigment in the testa, 

 for the seeds were either self-coloured (two plants of Th. 7) or spotted 

 and irregularly flaked (three plants of Th. 10). The amount of seed 

 produced by each plant was small, and as the spotted seeds of coloured 

 varieties of Pisum not infrequently "fluctuate" to heavy flaking or 

 self-colour, it was assumed to be a coincidence that all the seeds of Th. 7 

 were self-coloured. Later, however, the difference was discovered to be 

 the consequence of the action of a dominant factor, borne by Th. 7 but 

 not by Th. 10. It has since been found that the two "Unes" also differ 

 in chromosome structure, for Th. 7 is capable, on crossing, of giving rise 

 to the chromosome ring already recorded, whereas from the same crosses 

 with Th. 10 no ring is formed, and no sterihty has been observed. 



On making inquiries as to the original source of the Thibetan peas, 

 I was informed that Prof. Engledow had obtained them from Prof. John 

 Percival, who tells me that they were probably collected by members of 

 the Mt Everest Expedition of 1922. The seed he received consisted of a 

 mixture of forms, from which he selected three for further study, and 

 of these he has kindly sent me seed during the past season. One of the 

 three has a self-coloured testa, as I was glad to find, for I had lost the 

 pure stock of Th. 7 in 1926; but whereas Th. 7 had a dark hilum. Prof. 

 Percival's form has a light hilum. Thus a third varietal difference has 

 been found in the Thibet material. 



Description of types. 



The famihes (two) of Th. 7 were sown, Nos. 615 and 616; those of 

 Th. 10 (three) Nos. 617, 618 and 681, in 1926. The plants raised from all 

 the seed were fairly uniform, short (about 3 feet), ovnng to their Hmited 

 period of growth, but not true dwarfs, for the internodes were long (about 

 10 cm.). The stems were wiry, often branched; the flowers small, coloured 

 purple but of a pinker shade than is common. In time of flowering the 

 variety is not exceptionally early, the first flower being formed at about 

 the same time and place (about the 12th node), as in our cultivated 

 "2nd early" varieties; but soon after flowering begins they turn yellow 

 and ripen off. In consequence of the short growing period the amount of 

 seed produced is small, and only in favourable seasons are more than 

 five or six pods obtained, each containing about eight small seeds. The 



