NOTES AND ABSTRACTS. 



227 



gigantea — a tree that has attracted a good deal of notice on this side of 

 the Atlantic. The discovery of the original trees, places where they 

 are found, the immense size to which they have attained, and methods 

 of conversion are all fully explained, while the beautiful illustrations 

 serve a most useful purpose in elucidating the text. — A. T>. W. 



Bindweed, The Eradication of. By H. E. Cox {V.S.A. Dep. 

 Agr., Fanners' Bull. 368, Aug. 1909; plates). — This name has been 

 applied to several species of the Convolvulus family, which are very 

 aggressive weeds, doing much harm over a wide area to cultivated 

 crops and occasionally in orchards. The best method of dealing with 

 it is to keep down the top growth and thus starve out the under- 

 ground parts, and this is best done in three ways — by clean cultivation, 

 by lucerne> growing, and by hog pasturing. 



Other methods have been tried, but so far not with much success. 

 Figures are given of types of weed-cutters which have been found 

 useful in exterminating bindweed in America.— M. L. H. 



Biological Studies on Three Species of Aphididae. By J. J. 



Davis {U.S.A. Dep. Agr., Bur. ofEntom., Tech. Bull. 12, part 8; Feb. 

 1909). — Studies of the various stages, times of appearance, and so on, 

 of three species of Aphis which attack maize and sorghum are given 

 at length, together with a bibliography of each species. — F. J. C. 



Birch Stem, Sap-pressure in the {Bot. Gaz. vol. xlvhi. 

 December 1909, pp. 442-458; with 5 figs.). — Messrs. H. E. Merwin 

 and Howard Lyon found that the sap-pressure in both birches and 

 maples increases rapidly in amount from the morning until midday, 

 or shortly afterwards; it then slowly declines until sunset, after which 

 a gradual rise is maintained during the night. The sap-pressure is, 

 however, extremely sensitive to sunshine; it drops suddenly if a cloud 

 obscures the sun, and rises again in sunlight. The maple is not 

 nearly so sensitive. The highest pressures observed were 91 cm. 

 (1'2 atmosphere) in a birch 7' 5 cm. diameter, and in another 35 cm. 

 diameter and about 20 m. high 204 cm., corresponding to 2*68 atmo- 

 spheres. Such a pressure would support a column of water 7"8 m. 

 higher than the tree. These high pressures occur when the buds 

 are beginning to unfold, and no pressure is found in spring till the 

 ground has thawed considerably. 



By a series of ingenious experiments and calculations the authors 

 found that the evaporation from one tree amounted to about 480 ccm. 

 a day; that the duct space in a white birch, 11 cm. in diameter, 

 amounted to about 6,800 ccm., and that about 1*3 to '7 per cent, of 

 this internal duct space was occupied by gas bubbles. 



The authors calculated also the amount of expansion of the wood 

 due to a rise in temperature, and find that this expansion produces 

 the rise in pressure from sunrise to noon, as well as the oscillations 

 noticed in sunshine and cloud alluded to above. The ratio of expansion 



Q 2 



