228 PROF. R. J. HARVEY GIBSON ON THE AXILLARY 
The Axillary Scales of Aquatic Monocotyledons. By R. J. 
Harvey Grsxon, M.A., F.L.S., Professor of Botany in the 
University of Liverpool. 
[Read 6th April, 1905. | 
(Prates 5 & 6.) 
Dunmiwe the course of an investigation into the anatomy of 
certain species of Selaginella (1), I had occasion to compare the 
origin and development of the ligule of that genus with that of 
Isoëtes, and suggested a possible function for the structure in 
question. The ligule appeared to me to be of the nature of a 
specialised ramentum, such as one finds so frequently in Filicinez, 
protecting and keeping moist the young leaves and growing apex. 
This view was based on the contents of the cells of which it was 
composed, the large vascular supply and its history in relation 
to the leaf with which it 1s associated. 
That the ligule is of considerable phylogenetie importance 
I think cannot be doubted, although its functional importance 
in recent forms may perhaps be regarded as secondary. Its 
occurrence in fossil Pteridophyta seems to have been widespread, 
as shown by the researches of Hovelaeque (2), Maslen (3), 
Scott (4) and others. In view of the frequently expressed 
suggestion that the aquatie Monocotyledons may be looked upon 
as modern representatives of the more primitive Angiosperms, 
and that these in turn may have been genetically related to the 
ancestors of such types as Zso£fes, it occurred to me that it might 
be worth while to enquire into the structure of any vestigial 
members present in aquatie Monocotyledons which might suggest 
affinity with such problematical ancestors. The axillary scales 
known to occur in many Helobiee presented themselves as 
appropriate subjects for investigation, and the present paper 
embodies the results of an examination both of the adult strueture 
and of the development of these struetures in the following 
genera :— 
Potamogetonaces: Zostera (Z. marina and Z. nana), Potamo- 
geton (P. perfoliatus and P. crispus), Ruppia sp. 
Aponogetonaces : Aponogeton distachyon. 
Juncaginacee : Triglochin (T. maritimum and T. palustre). 
Alismacese : Alisma (A. Plantago and A. ranunculoides), 
Sagittaria sagittifolia. 
