6o Scott and Brebner. — On the Secondary 
considerable interest. It is impossible to doubt that secondary 
growth in the Irideae has originated de novo , and probably at 
a comparatively recent period, after the Order had attained 
something like its present development and geographical 
distribution. 
In spite of this there is a remarkable general agreement 
between the process in Irideae, and that in the arborescent 
Liliaceae and in the Dioscoreae. But in these two groups 
also secondary growth must have started independently. We 
arrive then at the conclusion that a closely similar mode of 
anatomical development must have been separately evolved 
in at least three distinct groups of Monocotyledons — probably 
more. We thus find that the phenomena which we have con- 
sidered in this paper offer a striking example of homoplastic 
modification, i. e. of the origination of similar, and apparently 
homologous structures in groups of organisms which are 
phylogenetically distinct. 
It is very probable that the first origin of secondary growth 
may be taking place in some of the Monocotyledons at the 
present day, just as we find medullary bundles appearing in 
certain Dicotyledons as an individual peculiarity. From this 
point of view it would be very interesting to examine some of 
those species of Arts tea which are not shrubby, and to see 
whether their short stems show any indications of secondary 
increase. 
For our material we are indebted to the Director of the 
Royal Gardens, Kew ; to Prof. F. O. Bower, F.R.S., Regius 
Professor of Botany in the University of Glasgow; and to 
Mr. F. W. Burbidge, F.L.S., of the Trinity College Botanic 
Gardens, Dublin, to all of whom we tender our warm thanks. 
The investigation was chiefly carried on in the Huxley 
Laboratory for Biological Research, at the Royal College of 
Science, London ; it was completed in the Jodrell Laboratory 
of the Royal Gardens, Kew. 
