F. Keeble and C. Pbllew 49 



these two lialf-dwarf varieties are distinguished from one another by 

 two well-marked characters, namely, length of internode and thickness 

 of stem. Thus Autocrat, whose normal height is 3 — 4 feet, has thick 

 stems (with large fleshy foliage of a bluish green colour) and short 

 internodes of about 3 inches in length. Bountiful, whose normal height 

 is 3^ — 4 feet, has thin stems (with foliage smaller than that of Autocrat 

 and of a yellowish green colour) and long internodes (5 — 7 inches). It 

 may be noted incidentally that the rates of growth of these two varieties 

 are very different ; the growth in length of the axis of Autocrat being 

 markedly slower than that of Bountiful. For example, when Autocrat 

 and Bountiful are planted at the same time. Autocrat grows one foot 

 whilst Bountiful grows two. The slowness of growth in length is asso- 

 ciated with short internodes. The two varieties differ also with respect 

 to mode of branching. Autocrat forms three to five branches at or 

 near the ground-level. These branches develop at the same rate till 

 they and the main axis are about 2 feet in height and then one axis 

 takes the lead. Bountiful shows a less fixed mode of branching. 

 Among the 1909 plants, some branched at the ground-level (2 — 4 

 branches), others formed their first branches a foot or so above the 

 ground-level. Generally speaking, thick stem appears to be associated 

 with branching, and thin stem with single stem, at the ground-level 

 We are engaged in endeavouring to work out the anatomical bases for 

 thick as opposed to thin stem, and for long as opposed to short inter- 

 nodes, and the bearing of these factors on growth. 



We will now consider the factors, thickness of stem and length of 

 internode, in relation with stature. That these factors maybe taken 

 as valid representatives of those which determine height is evident 

 from the following considerations : — 



Fi plants, 7 — 8 feet high, have all thick stems with long internodes 

 (6 — 9 inches). If the factor for thick stems is represented by T, and 

 its allelomorph (thin stem) by t, and if the factor for long internodes is 

 represented by L, and its allelomorph (short internode) by I : then the 

 gametic constitution of Autocrat is Tl, that of Bountiful is tL, and 

 hence the gametic constitution of F^ = TtLl. We ascribe the great 

 height of F^ plants to the presence of the factors T and L and to their 

 dominance over t and I. The suggestion may be hazarded that the 

 greater height and vigour which the F^ generation of hybrids commonly 

 exhibit may be due to the meeting in the zygote of dominant growth- 

 factors of more than one allelomorphic pair, one (or more) provided by 

 the gametes of one parent, the other (or others) by the gametes of the 



Joam. of Gen. i 4 



