149 
cells, while S. proteus has two such layers and that S. Gaert- 
nerianum has two peripheral layers of ee cells, while 
S. Koelreuterianum has but one such lay 
Similar investigations on Cytisus oe and the supposed 
Crataegus-Mespilus graft-hybrids have shown that these are also 
peripheral chimeras with one or more cell- as of one species 
growing over a body of cells of the other spec 
These studies have made clear the ees nature of these 
ei plants and explain the various ae ena which 
th The frequent reversion of branches to one or the 
other of ce pure aoe stock is due eae to a readjustme 
of the cells in the growing point so that one kind of cell develops 
to the exclusion of the other and the chimeral relationship does 
not exist in such a branch. As has been noted, the seedlings 
obtained from peripheral chimeras do not behave like the progeny 
of sex-hybrids, but are pure for one or the other of the parents. 
S. tubingense, for example, gives seeds that develop into plants 
that are pure nightshade. It is a well-known fact that in these 
plants the cells which form the embryo of the seed originate 
in the subepidermal layer of cells. If, as in the case of S 
the embryo is pure nightshade. In the case of S. proteus there 
are two peripheral layers of tomato cells, the germ cells are pure 
tomato and the seedlings are tomato plants 
n the union of stock and scion in the ordinary graft there is 
rather a limited region of contact of the two kinds of cells. a 
sectorial chimera there is a much greater extent of contact and 
in the peripheral chimera there is a still greater amount of 
contact. In the tomato-nightshade periclinal chimeras many 
thousands of tomato cells and nightshade cells are joined side by 
side. This rather intimate association of two kinds of cells in a 
single oe leaf, flower and fruit is interesting in view of the 
mechanical and physiological interactions which may take place. 
While the evidence indicates that the two kinds of cells remain 
entirely independent in their essential eee characters, they 
do interact, producing cell complexes that are intermediate in 
character. Furthermore, a slightly different eae of the 
