Connecting Threads in Graft Hybrids. 2 i 7 
p. 14) and by W. Gardiner (6, p. 110; 7, p. 187) and A. W. Hill 
(8, p. 282). The opposite view is upheld by Kienitz-Gerloff 
(9, p. 40) and by Strasburger (13, p. 493). The latter devotes the 
greater part of a long paper on “ Plasmaverbindungen ” to the 
citation of evidence in support of his view. The question is there 
very fully discussed and the literature quoted. A great number of 
cases is mentioned in which connecting threads occur between cells 
which almost certainly have only a very remote genetic connexion, 
but in no instance can this be shewn to be quite conclusively so. 
Could cells be found which were linked by connecting threads but 
which were known to be of different genetic origin, a theory of the 
formation of connecting threads from the spindle-fibres exclusively 
could not be any longer upheld. Even before the recent views with 
regard to graft hybrids had been developed, attention had been 
directed towards grafts as possibly offering suitable material for an 
investigation. The point where a graft is united with the stock is 
a region where tissues of different genetic origin are in living 
continuity, but the differentiation between the tissues of stock and 
scion cannot be easy and observations of connecting threads recorded 
between such tissues can scarcely be accepted as conclusive. 
Strasburger (13, p. 584 et seq.) describes connecting threads formed 
between the tissues of Abies nobilis and Abies pectinata, in such 
circumstances, and gives a figure (13, pi. xiv, fig. 10), but Arthur 
Meyer (11, p. 105) records his inability to find any means of dis¬ 
tinguishing between the tissues of stock and scion. 
The artificial production of graft hybrids of Solanuni lycopersicum 
and Solanuni nigrum by Winkler (15, 16, 17, 18, 19) and the inter¬ 
pretation of them as periclinal chimasras by Baur (1, 2, 3) has 
supplied new material for the study of connecting threads between 
genetically unrelated tissues. There seems no reason to doubt 
that Baur is right in his explanation and that, for example, Winkler’s 
Solanuni tubingense is actually Solanuni nigrum in an epidermis of 
Solanuni lycopersicum, and that Solanuni Kcelreuterianum is Solanuni 
lycopersicum in an epidermis of Solanuni nigrum. The theory is also 
applied to the long familiar graft hybrids Cytisus Adami (4; 10, p. 
268) and Cratcegomespilus. 
The Present Research. 
The most suitable of the graft hybrids for the purpose of 
a histological examination seemed to be Cytisus Adami, Solanuni 
tubingense and Solanuni Kcelreuterianum, since in these the epi¬ 
dermis is the only layer of cells belonging to the one component, 
