Duplicate genes for capsule-form in Bursa hursa-pasUrris. 



99 



of the valves, while the capsules of B. Hecgeri are not at all inflated, 

 and are in consequence slender top-shaped, being circular in cross- 

 section. The valves of the B. Bursa-pastoris capsule drop off at matu- 

 rity, while those of B. Heegeri remain attached, the seeds of the 

 latter being set free by the rupture of the mid-region of the valves. 

 Associated with these externally obvious differences there are also 

 striking internal differences. The inflation of the valves in B. bursa- 

 pastoris permits the seeds to assume a form determined alone by internal 

 forces, the result being that each seed is a nearly perfect spheroid. In 

 B. Heegeri the seeds are so crowded together that they are forced to 

 assume various angular forms (see fig. 2). There are also important 

 histological differences in the walls of the capsules, as might be inferred 

 from the different l)ehavior of the valves at maturity. In B. Heegeri 



Fig. 2. Transverse section of the capsule of Bursa bursa- paslor is (A) and of B. Hecgeri {B). 

 X 18. Drawn by J. Marion Siiull from microtome sections. 



the walls of the capsules consist of 6 — 8 layers of relatively small, thin- 

 walled cells, while in B. bursa-pastoris the number of cell-layers is 

 about the same, or perhaps on the average al)0ut one less, but the cells 

 in the latter are noticeably larger, and the internal epidermis is modified 

 by a thickening of the cell-walls, so that it forms a single layer of 

 stereome covering the entire interior surface of the valve. It is to this 

 layer of mechanical tissue that the resiliency of the valve is due. There 

 is a definite articulation of the valves to the margin of the dissepiment 

 in B. bursa-pastoris, which is wholly wanting in B. Heegeri (Fig. 3). 

 All of these differences, both external and internal, go together and 

 appear to be the product of a single gene acting in conjunction with the 

 genotypic nucleus (XX)^) which is common to both B. bursa-pastoris 

 and B. Heegeri. It is conceivable that this series of associated char- 

 acters may be due to a system of coupled genes, but this can be 

 demonstrated only by the discovery of individuals in wliich one or more 

 elements of the complex have become detached from the rest. 



') .JoHA.NNsicN, \V., Elemente der 

 p. .■i04; 2nd ed., l!)ia, p. ;j87. 



exakten Krbliclikeitslelire. 1st ed., 1900, 



