September, 1912. 



KNOWLEDGE. 



341 



nodules have branched and reached their full size the bacteria 

 disappear from the bacterial zone, and the basal zone grad- 

 ually encroaches upon and finally replaces all the other zones. 

 Pure cultures of the bacteria were made, and were found to 

 be identical with Pseiidontoiiiis radicicola, the organism of 

 the root-nodules of Leguminosae. Young Myrica plants 

 grown in sterilised nitrogen-poor soil did not flourish unless 

 they possessed root-nodules : plants devoid of nodules, after 

 infection with a culture of Myrica nodule bacteria, developed 

 root-nodules and grew well. 



The nodules of Alntis and Elacagniis are modified lateral 

 roots, and are perennial structures showing dichotomous or 

 trichotomous branching. They are produced by the infection 

 of the root with the nitrogen-fixing organism Pseudomonas 

 radicicoUi. as in the case of Myrica, which enters the root 

 and propagates itself in the cortex as a rod-shaped organism — 

 in Blaeagnus it produces a definite zoogloea in the form of a 

 thread-like jelly in which the bacteria are imbedded. The 

 further development of the organism, in both A Inns and 

 Elaeagnits. gives rise to relatively large spherical bodies, 

 which multiply until they fill the entire cell ; under certain 

 conditions these bodies divide into two, then each divides 

 again, until they lose their identity and a group of bacilli 

 remains in their place. .Apparently Psciidoinoiias radicicola 

 is polymorphic, the bacillus (rod-like) and coccus (spherical) 

 types being different forms of one and the same organism. In 

 Elaeagnus the bacteria are found mainly in the region 

 immediately behind the growing-point ; in Alnus the bacter- 

 oidal tissue traverses the entire length of the nodule. In 

 Elaeagnus the food storage cells are found towards the base 

 of the nodule ; in 4/nH.s- there are zones of tissue concentric 

 with the endodermis ; in both, the endodermis performs this 

 function. The coccus is apparently correlated with scarcity 

 of available carbohydrate and change of environment ; it is 

 much more resistant to the influence of external agencies than 

 the rod-shaped form. The organism is capable of fixing free 

 atmospheric nitrogen when isolated from the nodules, and its 

 presence is undoubtedly beneficial to the plant. 



MOLDENH.AWER'S WORK: PLANT AN.ATOMV A 

 CENTURV AGO. — Probably few botanists have noted the 

 fact that this year marks the centenary of the publication of 

 one of the most important works in the history of this science. 

 After the appearance of the classical memoirs by Malpighi 

 (1675) and Grew (1582), which laid the foundation of vegetable 

 anatomy towards the end of the seventeenth century, very 

 little further advance was made in this branch of Botany for 

 over a hundred years. During the interval between Grew's 

 work and that of Moldenhawer, it is true, some interesting 

 contributions were made by Casper Wolff (1759), Hedwig 

 (1782). Mirbel (1802). Bernhardi (1805), and Treviranus 

 (1806). .\s Sachs justly remarks in his " History of Botany," 

 Bernhardi's observations are decidedly the best in the whole 

 period from Malpighi and Grew to Moldenhawer ; for instance, 

 he distinguished pith. bast, and vessels, regarding them as the 

 three chief forms of vegetable tissue, and he correctly explained 

 the structure of spiral and annular vessels. Treviranus dis- 

 covered the intercellular spaces in parenchyma, though he 

 thought they contained sap and even described its movement. 



Johann Moldenhawer (1766-1827) went far beyond his 

 predecessors and contemporaries in the study of plant struc- 

 ture, both as to his actual observations and the accuracy 

 of his interpretations. He first hit upon the happy idea 

 of isolating cells, vessels, and fibres from plants by 

 maceration in water, though he still relied upon some- 

 what primitive methods — for instance, he mounted delicate 

 microscopic objects in a dry state and simply crushed 

 or picked his preparations to pieces instead of cutting 

 sections and mounting them in water, though the import- 

 ance of the latter procedure had already been pointed 

 out by previous workers. His best microscope was an 

 English instrument made by one Wright, and gave a 

 magnification of about four hundred diameters. Besides 

 making good use of his maceration method, which enabled 

 him to study the forms of cells and vessels and to make out 

 the structure of these tissue-elements, and the sculpture on 



their walls, more accurately than had been done before, he 

 made the fortunate choice of working largely with the maize 

 plant, instead of using the complicated and difficult woody 

 stems which had so greatly puzzled the earlier investigators. 

 Moldenhawer laid chief stress from the first on the contrast 

 between the vascular bundles and the cellular ground-tissue, 

 and thus hit upon a fundamentally important fact which set 

 later workers upon the right track. Even more brilliant was 

 his study of the structure of the stem in Dicotyledons, to 

 which he next proceeded, for he was the first to sec that the 

 growth of the woody stem can only be understood from the 

 structure of the young parts of the stem, in which there is a 

 ring of isolated bundles. All previous vegetable anatomists 

 had adopted Malpighi's theory of the growth of woody stems 

 — ^that the outer layers of wood arose by transformation of the 

 inner layers of bast. Moldenhawer, after carefully comparing 

 the structure of young and old stems, rejected this view and 

 thus removed an ancient error which soon afterwards 

 disappeared from botanical literature. This service alone 

 entitles him to an honourable place in the history of Botany. 

 He was the first to demonstrate the real nature of the stomata 

 on the surface of leaves, showing that these are not simply 

 holes in the outer walls of the epidermis cells but apertures 

 leading into the interior of the leaf and surrounded each by 

 two guard-cells. 



Moldenhawer's magnum opus, and apparently his only 

 work, is a quarto of three hundred and eighty-four pages, 

 entitled "" Beytrage zur .Anatomie der Pflanzen," and contains 

 six plates of remarkably accurate and well-executed drawings. 

 It was published at Kiel, where he was Professor of Botany, 

 in 1S12. Like Mirbel and several other earlier workers in 

 this field, Moldenhawer had his drawings from the microscope 

 made for him by someone else (in this case by a lady friend), 

 on the ground that more correct and trustworthy figures 

 would be obtained if the observer employed other people's 

 eyes and hands, and thus eliminated prejudice and precon- 

 ceived opinion from the drawings. 



CHEMISTRY. 



By C. AiNswoRTH Mitchell, B..\. (O.xon.), F.I.C. 



TOXIC ACTION OF OIL PAINTS.— It has long been 

 recognised that painters working with white lead paint are 

 liable to suffer from headache and other specific symptoms, 

 which have been attributed to lead poisoning. In order to 

 ascertain whether any volatile emanations containing lead are 

 given oft" by such paints, a series of experiments has been 

 made by Professor E. C. Baly (/. Soc. Chem. Ind., 1912, 

 XXXI. 515). 



In the preliminary tests brass tubes about a foot in length 

 and an inch in diameter were coated on the inside with a 

 paste of white lead and linseed oil, and a photograph was 

 taken of the spectrum of the light transmitted through the 

 tubes. .A comparison of the result with the photograph of the 

 spectrum of the light used proved that volatile emanations 

 capable of absorbing light rays were emitted by the white 

 lead paste, but not by pastes containing zinc white, or basic 

 lead sulphate. In each case the tubes were gently heated 

 to promote the separation of volatile substances. 



Further experiments showed that the volatile compound was 

 given off by white lead paints in appreciable quantities at the 

 ordinary temperature, but no definite evidence of the presence 

 of lead in the emanations could be obtained, even by heating 

 four pounds of a stiff" paste of white lead and linseed oil in a 

 vacuum, and condensing the volatile products in a receiver 

 cooled with liquid air. 



Hence it appeared possible that the poisonous volatile sub- 

 stance might be due to the influence of the combined water in 

 the white lead upon the linseed oil, whereas basic lead sulphate 

 and zinc white, being anhydrous compounds, would not have 

 such an action. The fact that a mixture of lead hydroxide and 

 linseed oil had the characteristic odour of white lead paint 

 soon after mixing, and gave off the emanation more readily 

 than the latter, lent support to the view that the hydration of 

 the lead carbonate was a main cause of the trouble. In other 



