2 20 Margaret Hume. 
The negative result in the case of Solatium Kcebrenterianum 
does not however in any way invalidate the positive evidence yielded 
by Cytisns Adami and Solatium tubingense, since no connecting 
threads could be demonstrated anywhere, in tissues either related 
or unrelated. 
Conclusions. 
The principal conclusion to be drawn is, that if Baur’s hypothesis 
is true, and there is every reason to believe that it is, and that graft 
hybrids really are periclinal chimaeras, then there is no doubt that 
genetically unrelated tissues can be joined by connecting threads. 
The threads therefore arise secondarily, since it is to be supposed 
that the naked cytoplasm of the two components does not come 
into contact. At any rate it is clear that the threads cannot have 
arisen from spindle-fibres, since no nuclei of the two components 
have ever been sisters. 
The breaking of the threads right across the centre of the 
median node when the middle lamella is split, is rather suggestive 
of a dual origin of the threads, half coming from the cell on either 
side. The median node is the thickest part of the thread and so 
presumably the strongest, and unless there were a division or joint 
running across it, a break might naturally be expected to occur at 
one of the thinner parts on either side of the median node. It must 
however be recalled in this connexion that some doubt exists as to 
whether connecting threads in the mature plant-cell are ever 
absolutely continuous across the middle lamella (8, p. 282). 
The foregoing results do not in any way necessitate the sup¬ 
position that connecting threads are always secondary and cannot 
arise from the spindle-fibres, though that view is rather indicated, 
at any rate for ordinary ground-tissue, because the connecting 
threads joining cells of genetically related and cells of genetically 
unrelated tissues are to all appearances entirely the same. 
The research was begun while holding a Bathurst Studentship 
of Newnham College. The writer is indebted to Professor Seward 
for kind permission to work at the Botany School. 
Botany School, 
Cambridge. 
April, 1913. 
