NOTES AND ABSTRACTS. 
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leaves are covered with grey or white patches, and are brittle and easily broken. 
The under side is mealy. The presence of slender filaments is easily seen. 
S. £. W. 
Pear Thrips in California, Life-History and Habits of the. By S. W. Foster 
and P. R. Jones (U.S.A. Dep. Agr., Bur. Entom., Bull. 173 ; April 1915 ; 5 plates, 
14 figs.). — This minute insect, which until 1904 was unknown to science, is at 
present one of the most important insect pests with which growers of deciduous 
fruits in the San Francisco Bay region and adjoining counties have to contend, 
and each year the insect is developing an ability to subsist on other and new 
food plants. It has very few natural enemies. — V. G. J. 
Pear Trees, To make Fertile. By V. Enfer (Rev. Hort. vol. lxxxix. pp. 320- 
321). — To render pear trees fertile it is better to bud the vigorous branches 
with fruit buds taken from very fertile trees, than to make use of the doubtful 
expedient of root pruning. This operation should be carried out in August or 
early September. — 5. E. W, 
Peats and Humus Soils, The Ammoniaeal Nitrogen of. By J. C. B. Ellis 
and C. G. T. Morison (Jour. Agr. Set. vol. viii. Part 1 ; Sept. 1916). — In view 
of the attention now being given to the question of the reclamation of heath and 
moorland the determination of ammoniaeal nitrogen furnishes a figure of some 
importance. The authors have therefore examined a number of neutral and 
acid peats from Scotch and Irish deposits. Following Russell's method they 
distil the peat for some hours, at 40 0 C, under reduced pressure, with water and 
magnesium oxide. At the end of about two hours there is a marked drop in the 
amount of ammonia evolved, and the authors consider that at this stage 
practically all the ammoniaeal nitrogen has been evolved and that more stable 
nitrogen-containing substances are being attacked. Large quantities of ammonia 
may be set free in this way, as is shown by the following : NH 3 per cent, in 
ordinary arable soil -0003, in neutral peat "005 (sixteen times as much), in acid 
peat «oi (thirty-three times as much). Moreover, much of the ammoniaeal 
nitrogen is in a highly soluble form, for one-half to three-quarters of that evolved 
by distilling the peat with magnesia can be removed by merely shaking the peat 
with water.—/. E. W. E. H. 
Pecan Culture : with Special Reference to Propagation and Varieties. By 
C. A. Reed (U.S.A. Dep. Agr., Farm. Bull. 700, January 18, 1916). — The pecan is 
found only in certain parts of the United States and Mexico. 
The price of nuts is an exceedingly variable factor. A short time ago these 
nuts were little in use except at holiday times, and the prices to the producer 
were such that but a small portion of the crop was actually gathered, 3 cents 
a pound then being considered a fair price. 
The increased consumption since that time is largely due to the introduction 
and use of machines for cracking the nuts, and has caused a rise in price to a 
maximum of 25 cents a pound for the wild nuts. The average price is probably 
between 10 and 15 cents a pound. 
With reference to the pecan, the term " papershell " has been extended in 
its application until it is now practically without significance. Originally 
applied to those types of pecans having such thin shells that one could easily 
be cracked when two were crushed together in the hand, the term during recent 
years has been made to include all cultivated varieties ; many of them have as 
hard shells as the average wild nuts. Properly speaking, the term " papershell " 
never referred to a particular variety ; its correct application has been only 
with reference to varieties having very thin shells. — A, D. W. 
Permeability of Certain Plant Membranes to Water. By F. E. Denny 
(Bot. Gaz. vol. lxiii. No. 5, May 1917, pp. 373-397; 2 figs.). — In this paper 
investigations are described which were carried out to measure the rate at which 
water passes through known areas of certain membranes. Different species 
of plants and different membranes of the same species showed large differences 
in the rate of penetration. It was found that the seed coats of peanut and 
almond showed a difference in permeability to water in opposite directions 
through the membrane, the faster rate being from the external towards the 
internal portion of the seed. When solutions of varying concentrations were 
placed on opposite sides of the membrane, it was found that the relation between 
rate and concentration difference was complex, and that in general equal osmotic 
differences do not necessarily produce equal rates ; the rate is greatly affected 
by changes in the concentration of the internal solution : but no mathematical 
