712 



NATURE 



[August 5, 1920 



and instead of gun-cotton, a lower nitrate of cellu- 

 lose was used. The great factory at Gretna, also 

 built by Mr. Quinan, manufactured R.D.B. 

 cordite exclusively, and this soon became the only 

 propellant made in this country for the Land 

 Service, It was produced both by Government 

 and by private firms in enormous quantities. 



The need for ether and alcohol for this propel- 

 lant led to the restrictions imposed on alcohol. 



The recovery of the new solvent presented new 

 problems, and investigations on these were under- 

 taken, which have increased our knowledge of the 

 principles underlying the absorption of vapours. 



As difficulties arose in the gelatinisation of the 

 special nitrocellulose required for this powder, the 

 Department continued its studies on the viscosity 

 of cellulose and nitrocellulose with important 

 results, which formed the groundwork of the pro- 

 cedure adopted in supply for obtaining uniformity 

 in the cotton used in the nitration, and a dimin- 

 ished usage of solvent in the incorporation. 



Other Explosives. — Many other explosives for 

 special naval and land purposes were put forward 

 by the Department and adopted by the Service 

 after their properties had undergone investigation. 



Design of Ammunition. — A feature of the work 

 is the close connection between mechanisms con- 

 nected with ammunition and the utilisation in them 

 of explosives the properties of which have been 

 found specially suitable. The Department was 

 fortunate in the success which has attended its 

 percussion fuse (No. io6), which played such 

 an important part in the war. 



Pyrotechnics. — New demands occasioned by the 

 war led to the study of compositions for pyro- 

 technic and incendiary purposes and to chemical 

 investigations on the compatibility of the ingre- 



dients used. Many new compositions were de- 

 vised and adopted for signals, stars, and incen- 

 diary shell. 



Study of the Theory of Explosives. — The study 

 of the chemical constitution of nitro-compounds 

 has been referred to ; but a large amount of in- 

 formation the usefulness of which has been re- 

 flected on Service requirements has accrued from 

 the development of systematic work on such 

 subjects as the calorimetry of explosives, for 

 which new methods and apparatus have been 

 devised, their sensitiveness, their rates of 

 decomposition and of detonation, and the 

 pressure of the blow they develop. The last was 

 an extension of the work of the late Prof. B. 

 Hopkinson, and has been fruitful in advancing 

 knowledge of theory as well as in providing in- 

 struments for quantitative registration of the 

 effects of explosives contained in Service com- 

 ponents. 



General. — The high quality and elTiciency of 

 our ammunition, in spite of shortage and the need 

 for providing substitutes, have been obtained as 

 a result of the continuous application of chemical 

 and physical research. The research initiated 

 and carried out provided in numerous cases 

 methods for the production of explosives, and 

 demonstrated the conditions for their safe em- 

 ployment ; principles of fundamental importance 

 w^ere discovered which were utilised in the design- 

 ing of ammunition ; causes of failure at early 

 stages were discovered, thus avoiding unsatis- 

 factory issues of material ; and substitutes and 

 alternatives, without which some of our great war 

 manufactures could not have been carried on, 

 were sought and discovered. 



(To he continued.) 



The Earliest Known Land Flora. ^ 

 By Prof. F. O. Bower, F.R.S. 



II. 



COMPARISON of these four fossil species from 

 Rhynie with other fossils already known 

 from the early Devonian period shows that a very 

 homogeneous flora existed at that time, consisting 

 chiefly of leafless and rootless land-living plants. 

 These and other characters, such as their large, 

 distal, sometimes solitary, and often forked 

 sporangia, stamp these plants as exceptionally 

 primitive. Among living plants the nearest of 

 kin to them are clearly the Psilotaceae, a family 

 which has long presented a problem in morpho- 

 logy and classification. It comprises two living 

 genera, Psilotum and Tmesipteris. Both genera 

 are rootless. Their imperfect morphological 

 differentiation is shown by the fact that botanists 

 are not yet agreed whether their lateral appen- 

 dages are to be held as truly foliar or not. Psilo- 

 tum is native throughout the tropics, and is repre- 



1 Discourse delivered at the R lyal Institution on Friday, April 30' 

 Continued from p 684. 



NO. 2649, VOL. 105] 



sented by two well-marked species. The com- 

 monest, P. triquetrum, has upright and shrubby 

 aerial shoots, with radial construction and fre- 

 quent bifurcations. These spring from leafless 

 underground rhizomes, profusely bifurcated. Thev 

 are covered with rhizoids, and contain a mycc- 

 rhizic fungus. On the lower part of the aerial 

 shoots simple spine-like leaves are borne, but 

 towards the distal ends these are replaced by 

 forked spurs, between the prongs of which a 

 synangium, usually with three loculi, is seated. 

 The aerial shoot is traversed by a vascular strand 

 consisting of xylem in the form of a hollow many- 

 rayed star, with sclerotic core, and branch-strands 

 run out to the appendages. The whole is covered 

 by epidermis with stomata, and the cortex pro- 

 vides the photosynthetic tissue. Tmesipteris is 

 represented by only one species, limited to Aus- 

 tralasia. It grows usually among the massed 

 roots that cover the stems of tree-ferns, but some- 

 times upon the ground. Its general form is like 



