Anatomy and Development of the Stem in Pteridophyta, &c. 119 



trations required for simple molecular combinations only of nitric 

 acid (HN0 3 ) with water. Some of these combinations have been 

 alluded to by previous writers on this and kindred substances, 

 while others have either not been examined or have formed the 

 subject of a diversity of opinion. 



In the case of the densities and contractions the best denned 

 points of alteration correspond to the composition of the hydrates 

 HN0 3 with 14, 7, 4, 3, 1*5, and 1 molecular proportions of water 

 respectively, while in the case of the refractive indices, apart from 

 consideration of density, the most marked points correspond to the 

 14, 7, and 1-5 hydrates. In the case of the contractions and electric 

 conductivities, but to a less degree in that of the refractiA r e indices, 

 there is remarkable discontinuity at concentrations 95 to 100 per 

 cent., which can possibly be explained rather by some decomposition 

 than by some combination. 



Further, the more detailed experiments on the contractions show 

 that these points of discontinuity, though to some degree real, yet to 

 another degree are ideal, in that an apparently abrupt change of 

 events is resolved into either a gradual transition or a transition stage, 

 which differs from either the preceding or successive phase. In the 

 present state of knowledge it appears difficult to interpret the course 

 of events between each phase, as there are the possibilities of at least 

 two cases involving two entities, namely, (i) combined acid and water 

 -with free water, and (ii) combined acid and water with free acid. 



Investigations into a possible case not readily provided by nature or 

 art, of two liquids which rapidly mix but slowly combine each with 

 each, might solve many of the problems of solution towards which the 

 present memoir may serve as a slight contribution. 



" The Anatomy and Development of the Stem in the Pterido- 

 phyta and Gymnospernis." By EDWARD C. JEFFREY, Ph.D., 

 Lecturer in the University of Toronto. Communicated by 

 I). H. SCOTT, F.R.S. Received July 15, 1901. 



(Abstract.) 



Comparatively little attention has been directed to the subject of 

 the development of the stem. This research concerns itself chiefly 

 with the development of the cauline fibro-vascular skeleton, since this 

 appears to be most interesting from the phylogenetic and morpho- 

 logical standpoints. A study of numerous examples drawn from the 

 main groups of the Pteridophyta and Gymnospernis has led to the 

 conclusion that the polystelic type of Van Tieghem does not originate, 

 .as he states, by the repeated bifurcation of the epicotyledonary central 



K 2 



