4o8 



N.<^TURE 



[May 2'], 1920 



stars than the remaining parts of the sky — a theory 

 upon which some doubt had been cast by earlier work 

 of this nature. The method, although of extreme 

 simplicity, has certainly proved efficient for the first 

 of these objects, and various systematic errors of scale 

 have been clearly exhibited. With regard to the second 

 object, an examination of the ratio of the number of 

 faint stars to bright in the various regions investigated 

 appeared at first to negative Prof. Kapteyn's con- 

 clusion ; but, although this ratio was not found to 

 vary with galactic latitude, certain changes were 

 detected in different parts of the sky. Prof. Turner 

 has thus been led to the interesting conclusion that 

 regions of "obscuration" exist which tend to obliterate 

 the fainter stars, and these regions appear to form a 

 spiral in the heavens, the central line of which is 

 approximately given by the equation 



+ 3-668 = 247°, 

 where a denotes right ascension and 8 declination. 

 There appears to be a fairly sharp boundary to this 

 '"spiral of obscuration" on the side of smaller R.A. 

 -in the northern hemisphere, and on the side of 

 greater R.A. in the southern hemisphere. 



Another very valuable piece of work is represented 

 by a series of papers on "Baxendell's Observations 

 of Variable Stars," edited by Prof. Turner and Miss 

 -M. A. Blagg. A very considerable amount of pains- 

 taking work must have been expended on this task 

 of revising and editing Baxendell's observations of 

 some twenty-three long-period variables. The greater 

 part of the work appears to have been done by Miss 

 Blagg, and the result as a whole is certainly a most 

 valuable contribution to the study of this particular 

 branch of astronomy. In connection with this sub- 

 ject we may also mention two papers by Prof. Turner 

 "On the Classification of Long-period Variables," in 

 which the alternative classifications of the author and 

 of the Rev. T. E. R. Phillips are discussed and com- 

 pared at some length. Both methods are considered 

 to be useful, and a suggestion is made that some 

 stars might pass from one of Phillips's groups to the 

 other during the course of their evolution. This latter 

 idea is more fully discussed in the particular case of 

 W Cygni, which appears to be changing froni 

 Phillips's Group I. to Group II. 



There are many other shorter papers of consider- 

 able interest, but these are too numerous to be noted 

 here individuallv. The whole collection pays ample 

 tribute to the energy and resource with which work 

 has been carried out at the observatory during the 

 trying period of the last few years. Apart from the 

 many difficulties directly resulting from the war, there 

 have been other troubles with which the staff has had 

 to contend. In particular, we regret to note the 

 decease of the caretaker, Mr. J. Mullis, who had been 

 with the observatory since its erection in 1874. There 

 is at present no second assistant or resident com- 

 puter, and Prof. Turner and his staff must certainly 

 be congratulated on the way in which the work has 

 been carried on in the face of these and numerous 

 other difficulties. D. L. E. 



The Alligator Pear. 



THE cultivation of the Avocado or alligator pear 

 is the subject of articles by Mr. W. G. Freeman 

 and others in the Bulletin of the Department of Agri- 

 culture, Trinidad and Tobago (vol. xviii., part 3). 

 The Avocado {Persea gratissima), a member of the 

 family Lauraceae, is a pear-shaped fruit with a large 

 central stone, the amount of covering flesh varying 

 considerably according as the kind is good or poor. 



NO. 2639, VOL. 105] 



It is one of the most important of the fruits which 

 have become widely distributed since the discovery 

 of the New World. It is probably a native of tropical 

 America, and was introduced at an early date into the 

 West Indies, where it is now naturalised. Sir Hans 

 Sloane, in his "History of Jamaica" (1707-25), gives 

 a. long description of the tree and its fruit, and Dr. 

 Patrick Browne (1756) is eloquent on the flavour of 

 the latter and the esteem in which it is held. The 

 edible portion of the fruit varies from a little under 

 one-half to more than three-quarters of the weight 

 of the whole, according to the thickness of the rind 

 and the relative size of the seed. Its food-value is 

 mainly due to its high fat content, which in some 

 varieties approaches that of the olive, and is esoecially 

 high in the fruit grown in Florida and California. 



Although so long cultivated in the W'est Indies, yet 

 little attention has been given until recently to the 

 selection and propagation of good varieties. It is an 

 extremely variable plant, and the method of selecting 

 seeds from trees bearing the best fruit and of high 

 productiveness gives uncertain results, as the varieties 

 do not come true from seed. But by budding- or 

 grafting from good varieties these may be fixed, and 

 by this means poor trees will be converted into good 

 varieties. Mr. Freeman suggests the probability of a 

 seedless Avocado being obtained, as occasional seed- 

 less fruits have been reported from the United States 

 and Honolulu. The Avocado needs no very special 

 care in cultivation, and does very well on the poor 

 soil of parts of the northern range in Trinidad. 

 Budding has been practised at the St. Clair Experi- 

 ment Station for the last four years, and the curator, 

 Mr. R. O. Williams, gives details of the operation. 

 The method is the same as that generally adopted for 

 roses and citrus. The full-grown tree is fairly free 

 from insect pests, but the plant is more susceptible in 

 early stages and when recently budded. Mr. F. W. 

 Urich describes the various insect pests and means 

 for combating them. A more serious disease which 

 attacks the fruit is the so-called anthracnose, ;erv 

 closely related to the fungus which causes anthracnose 

 of the mango. In the case of fruits packed for export 

 this disease causes complete rotting of the whole 

 consignment. Repeated sprayings with Bordeaux 

 mixture are necessary to prevent its development. 



The Improvement of Grassland.^ 



IT is too often the case that grassland is left to 

 take care of itself, and that no steps are taken 

 for, its improvement. Even where manuring is carried 

 out it is usually confined to occasional dressings of 

 farmyard manure ; little or no use is made of artificial 

 fertilisers, and the beneficial effects of lime upon the 

 herbage are far less widely known than they should 

 be. The consequence is that much of the finest 

 pasture and meadow land in the country is carrying 

 only a second- or third-rate herbage simply from lack 

 of knowledge of the most effective treatments to bring 

 about improvement. For the education of public 

 opinion in this respect nothing is more useful than 

 demonstration plots, and the Ministry of Agriculture 

 and Fisheries has issued a most valuable and com- 

 prehensive pamphlet outlining schemes of experi- 

 ments suitable for this purpose. The schemes in- 

 tended for farmers are simple in character and direct 

 and practical in their object, while those drawn up 

 for the agricultural colleges and institutes deal with 

 experiments requiring considerable attention and 

 supervision. 



I " The Improvement of Grassland : Suggestions for Demonstrations and 

 Experiments." Miscellaneous Publications No. 25. Ministry of Agriculture 

 and Fisheries. 



