A. H. Trow 265 



Observations on Stem Colour. 



At an early date in the course of these experiments it was noted 

 that some of the pure tyj^es were green-stemmed, viz. erectus, erect us, 

 radiatus and multicaulis; others had stems which were more or less 

 reddish in colour, viz. praecox, genevensis and lanuginosus. No attempt 

 was made to study the behaviour of this pair of characters in segregation 

 until 1911, when certain families of the F^ generation of the crosses 

 lanuginosus x erectus and lanuginosus x multicaulis were found to consist 

 either of green-stemmed individuals only, of red-stemmed individuals 

 only, or of individuals of several types of stem-colour. It seemed that 

 stem-colour varied independently of the other characters under investi- 

 gation, and was therefore probably represented by definite factors in the 

 constitution of the plants. 



There are very great difficulties in arranging the plants in definite 

 categories according to stem-colour, for green-stemmed plants tend, 

 under certain conditions (exposure), to develop some red colour, and 

 red-stemmed plants lose some of their red colour in shade. Moreover, 

 the red-stemmed types are not red-stemmed throughout — the upper 

 internodes are apt to be green, although these parts are subject to 

 the most intense illumination. Green-stemmed plants, if they develop 

 red colour at all, do so most freely on the lowest internodes. Let GG 

 denote red-stemmed and gg green-stemmed plants. When G is present 

 (GG) and certain standard conditions are maintained, at least one-half 

 of the main axis should be of a reddish colour ; if (? is absent (gg), 

 there should be little more than traces of red colour except at the very 

 base of the main stem ; the intermediate condition (Gg) is represented 

 by varying degrees of redness, both as regards the intensity of the 

 colour and its extent. The pure types grown in adjacent beds under 

 similar conditions furnish a convenient standard of reference. 



Perhaps the simplest proof of the existence of factors for stem colour 

 is furnished by the investigation of the i^, generation of genevensis 

 X erectus, radiatus. Both parent plants are glabrous, but differ with 

 respect to two pairs of characters. Genevensis is red-stemmed and non- 

 radiate (GGrr) ; erectus, radiatus is green-stemmed and radiate (ggRR). 



Exp. 16. A colony of 234 plants was raised and brought to the 

 flowering stage without the loss of a single plant. 



