S50 SCIENCE PROGRESS 



those which are not. In the latter, the lattice is molecular, and 

 certain physical constants, such as heat of transition, are less 

 than in the case of those compounds which have a lattice 

 composed of ions. A. Quartaroli {Gazzetta, 50, ii, 60, 1920 ; 

 Jour. Chem. Soc, 118, ii, 602, 1920) discusses the bearing of 

 X-ray work on crystals on the Law of Definite Proportions, and 

 comes to the conclusion that in minute crystals, where the 

 number of atoms is small, the composition may vary from the 

 recognised formula. As such " embryo crystals " with only a 

 few atoms would be submicroscopic, it seems better not to class 

 them as true crystals, particularly as they cannot be directly 

 examined. 



F. M. Jaeger {Proc. Akad. Amsterdam, 22, 815, 1920) has 

 investigated the patterns obtained by passing a beam of X-rays 

 through lamellse of mica symmetrically superimposed, and finds 

 that dextro- and laevo-rotatory aggregates give the same 

 pattern. L. Vegard {Ann. Physik (4), 63, 753, 1920) reiterates 

 his views on the role of the water of crystallisation in the 

 structure of the alums. The relations between cleavage and the 

 lattice structure as determined by X-ray methods is discussed 

 by R. Scharizer [Zeit. Kryst. Min., 55, 440, 1920). 



BOTANY By E. J. Salisbury, D.Sc, F.L.S., University College, London. 



Ecology. — An interesting study of the conditions above and 

 immediately below the tree limit on Mount Marcey, New York, 

 has furnished data showing the effectiveness of the forest 

 shelter (Adams, Barnes, Hankinson, Moor, and Taylor, Ecology, 

 1920). The timber line is situated at 4,900 ft., or 444 ft. below 

 the summit. The solar radiation, at the ground-level, in the 

 dense dwarf forest of Abies balsamea just below the timber- 

 line, was only 4 per cent, of that above, whilst in the tree 

 crowns it reached 89 per cent. At the lower limit of the forest 

 (4,250 ft.) the radiation was 51 per cent. The evaporation 

 exhibited a similar minimum just below the timber-line, despite 

 the general increase with altitude. The temperature gradient 

 showed an abrupt fall above the tree limit, and it is suggested 

 that this is one of the most important factors causing the 

 abrupt change in vegetation. It is significant that the rate of 

 growth, which, like the longevity of the trees, dimmishes with 

 altitude, falls abruptly at the upper tree limit. 



A. Tengwall has written an extensive account of the vegeta- 

 tion of the Sarek region of Lapland {Naturwiss. tint, des 

 Sarekgebirges im Schwedische-Lappland, 1920) mostly of a 

 descriptive character. The chief type of woodland is dominated 

 by birch, which attains an altitude of from 533-832 m. on 

 southern aspects, and from 590-760 m. on northern. The 



